Here's an interesting thing to explain!
Today, Theresa May is 'reshuffling' her Cabinet, something that PMs do relatively often in minor ways, but quite rarely in major ways (often after elections). Today was meant to be major, but her position has been so weakened that she's been forced to keep the major figures in place. Reshuffles are much more complicated than in a Presidential system, because almost all the cabinet are always MPs, so there's a finite number of people to choose from. So, instead of picking and choosing each post one by one from the populace as a whole, the PM has to move around her stock of high-status MPs, with every person leaving one job having to be replaced by someone else, who is usually in turn leaving another job, etc etc. Like reshuffling a pack of cards. Ministers usually aren't in any way experts in their subjects - although if a minister does have outside experience, that can help them in getting certain jobs (like, people with some military experience are more likely to get the MOD, though it's not actually required; women often get Education or Health or the like, although that's beginning to change*).
[NB although we talk about cabinet reshuffles, in the UK we distinguish between three things: government ministers, who form "the government" and each have particular tasks; the party in power, which consists of about 1/3 government ministers and 2/3rds "backbenchers", who are expected to support the government but aren't directly part of it because there aren't enough jobs to go around; and cabinet ministers specifically, who are the top twenty or so ministers.]
*One infuriating thing the PM did during this campaign: she and her husband went on a gentle interview show and explained that he took the bins out every week because "there are boy jobs and girl jobs, you see". This from a female Prime Minister, former Home Secretary, who has a female Home Secretary herself (HS being about the most 'boy job' that you can get, traditionally - prisons, police, terrorism, etc). Not helping.
But that's not what I want to explain! You probably knew all that. No, what I want to talk about is the appointment today of Damian Green as First Secretary of State and Minister of the Cabinet Office. Because that's actuall an interesting title!
OK, so what we need to know to start with is that much of the UK government is, constitutionally, fictional. Constitutionally, there is no such thing as a "prime minister", for instance. There have been PMs for hundreds of years, but it's only in the last century that they've started to get one or two minor legal acknowledgements - they're now in the ritual 'order of precedence' for entering formal banquets, for instance, and there are one or two laws that mention them - but they still have no official power or importance. If you go to No. 10 Downing Street, for instance, you will not find "Prime Minister" written on the door! 10 Downing Street is the residence of the First Lord of the Treasury. And that's a double fiction - first, because "First Lord of the Treasury" has had no practical importance for three hundred years (with a few exceptions, the last of which was a century ago, the FLotT has always been the PM); and, second, because, in keeping with our tradition of fictions, the PM hasn't actually lived at 10 Downing Street for twenty years now (well, Brown lived there very briefly for part of his reign). She actually lives at 11 Downing Street, the official residence of the Second Lord of the Treasury (who, by convention, is almost always the Chancellor of the Exchequer).
The country is legally run by the Cabinet, of which the First Lord is only one member. And most people think that the Cabinet is the upper echelon of the ruling party, or the advisors to the PM - both are legally false. Instead, the Cabinet is the executive committee of the Privy Council, a body that includes members from all the major parties and beyond, and which (via its executive cabinet committee) rules the country, although most people haven't heard of it.
Why does the PM bother with the title "First Lord of the Treasury"? Partly tradition and to avoid having to change the name on the plaque. But mostly because the country is run by Cabinet, and "PM" is not a Cabinet position. First Lord is, however. It is one of several old positions that continue to be used only because they come with membership of cabinet... and a salary. Other examples includes Chancellor of the Duchy of Lancaster (who spends one day a week overseeing some of the Queen's estate management) and the Paymaster General (who sort of oversees government payments, although it's all done by professionals these days). And Lord Chancellor. Lord President of the Council. Lord Privy Seal. Note that most of those jobs are neither held by aristocrats nor by members of the House of Lords. [hence a famous description of the Lord Privy Seal: neither a Lord, nor a Privy, nor a Seal.]
So, for example, Natalie Evans actually is a Lord - she's been appointeed as a Baroness, and currently serves as Leader of the House of Lords (unless she's been reshuffled this afternoon!) - which means that she orchestrates the passage of government legislation through the Lords (and also has some impartial administrative functions for the House of Lords as a whole). However, "Leader of the House of Lords" is not a Cabinet position, and does not legally have any salary attached to it. So Ms Evans is also the Lord Privy Seal, the woman in charge of administering the monarch's personal privy seal (as opposed to her great seal of state, administered by the Lord Chancellor). LPS is a Cabinet position, with a salary, but has a lot of free time for doing things like being the leader of the house of lords, because the monarch's privy seal hasn't actually been administered for several centuries, and quite possibly doesn't actually exist.
It's a make-do-and-mend approach to government. As old jobs become obsolete, we re-use the titles and rituals and whatnot for new purposes, rather than going to the bother of changing all the letter heading and the like every time a new important job comes along...
[Oh, and "First Lord of the Treasury"? There's another fiction there. The actual chief minister of the crown is the Lord High Treasurer. But there hasn't been a Lord High Treasurer since 1714. The holder of the post had become too powerful, so he stopped being appointed. Instead, in his absence, the Treasury is administered by a commission of ministers. The First Lord Commissioner of the Treasury has ended up running the country, while the Second Lord Commissioner of the Treasury has been deputised to actually run the Treasury (and is better known by his other title, Chancellor of the Exchequer).]
So, back to the subject. What does the First Secretary of State do? One of three possible things. But to answer that we need to look at another job: Deputy Prime Minister.
DPM is a job that's been around since WWII. The DPM is second only to the PM. Crucially, the DPM has no right of succession, as with a Vice-President or the like; they do, however, speak for the PM in certain situations (so when the PM is out of the country, for instance, the DPM is officially in charge, chairs the weekly meetings, shows up at PMQs, etc). [PMQs - when the PM is asked questions by the Commons every week, broadcast live. Mostly pointless theatre.]
Now, like the "PM", the DPM is not a member of Cabinet ex officio. Originally that didn't matter, because the DPM was always another cabinet minister at the same time. However! What if the DPM doesn't have any job?
This happened a few times back in the sixties, and a solution they devised was the title "First Secretary of State". The First Secretary of State is, by definition, a Secretary of State, which means he's a member of Cabinet - he gets to sit in Cabinet and he gets a salary. And as his name suggests, he's first among all the secretaries of state (other than the PM). So a DPM who had no other Cabinet job could be made First Secretary of State (rather than giving him a more menial job like Lord Privy Seal, which has no recent tradition of authority or power).
This then fell out of use for several decades, before being used twice more: for Michael Hesseltine and for John Prescott. Regarding the latter case, which I know better: Tony Blair's Deputy Prime Minister was not, as you might think, Gordon Brown, the Chancellor, although he was very much the de facto second-in-command and obvious successor. Instead, the DPM was John Prescott, a representative of the 'Old Labour', working class, union-affiliated left of the party. This is often how DPM is used - not someone with real power, but either an advisor and supporter to the PM (like Whitelaw and Howe under Thatcher) or as a symbolic sop to a faction that otherwise lacks power, like Prescott under Blair. [only two DPMs have gone on to be PM, and one of those was due to a wartime coalition arrangement]. In 2005, however, Blair and Brown got sick and tired of Prescott not being very good at anything, and sacked him from all his jobs other than DPM. To keep him in the Cabinet, and to reassure his supporters that he still had authority, they made him First Secretary of State.
So that's one thing FSoS can do. It can be a back-up title for the DPM, and in a way a form of political emmasculation: it allows a PM to give someone the official trappings of being DPM, while removing from them any specific ministerial responsibilities (which could act as a power base for them).
But in 2009, Gordon Brown, his party collapsing around him, came up with a new idea. To effectively fix everything for him, he invited Peter Mandelson, an architect of the Blair regime, back into government. Mandelson wasn't an MP, so he was appointed a Lord, and to put him into Cabinet he was made First Secretary of State. But why? Well, everybody hated Mandelson, including Brown, and the public, and the idea of taking this guy whom everyone hated, and who wasn't even elected, and making him "Deputy Prime Minister", would just have been politically suicidal. Instead, he was simply made First Secretary of State, a title that told insiders that he had the authority and rank of a DPM but told outsiders "don't worry, he's not really the DPM!". As it happened, this was basically the first time the general public noticed the title, and it kind of backfired, making him sound like a sort of macchiavellian prince.
So that's a second thing it can do: it can again emmasculate, by elevating one person ahead of everyone else, but withholding the title "Deputy Prime Minister", which would give them power and public profile, and arouse the envy of others.
But in 2010, the new PM, David Cameron, picked up the "First Secretary of State" idea and gave it a third spin. Cameron was in a coalition government, and as a result he was forced to make the leader of the junior party, the Liberal Democrats, the Deputy Prime Minister. But obviously, titles aside, there was no way a Libdem would be accepted as #2 in the government by the larger party. At the same time, just as Blair didn't dare make Brown DPM, so Cameron didn't dare give his actual #2 and chancellor, and expected successor, Osborne, any official elevated position, since that would be crowning him "guy who's going to stab me in the back and take my job". Instead, Cameron made the Libdem leader his Deputy Prime Minister, but ALSO appointed William Hague, a somewhat older politician who had gravitas but no ambition to replace him, as First Secretary of State. A sort of way of telling the Tory rank and file "we had to make Nick Clegg DPM, but if we hadn't had to do that then this guy would have been my deputy".
[so how did Clegg sit in Cabinet? By being made Lord President of the Council.]
So there again it emmasculates the DPM, by giving him a rival who has effectively the exact same job...
Then, in 2015, after the coalition, when Osborne had grown more powerful and Hague less so, Cameron felt he had to give a deputy position to Osborne. But he didn't want to make Osborne DPM, which might be too powerful, so instead he made him FSOS - that second approach to the position again.
[also, osborne probably didn't want DPM. There's a general theme in British politics that appointed heirs rarely inherit. DPMs don't become PM, leadership frontrunners lose leadership elections, and people who remove PMs never replace them ('the hand that wields the knife never holds the crown'). So people with ambitions often try to avoid being marked out as a frontrunner or a designated successor. FSOS gave Osborne authority, but didn't put as big a target on his back as DPM would have done.]
Now, May didn't bother with either a FSOS or a DPM at first, because she was our Glorious Leader who needed no help. Now she's made Damian Green (a wanker) FSOS. On the one hand, this placates her rank and file, who were worried about her becoming isolated - "look, I'm appointing a FSOS to support me!". On the other hand, it helps guard her back - Green is an old ally of hers, and him being there as ostensible #2 helps stop other ministers competing for that #2 spot. On the third hand, NOT making him officially DPM protects her against him - FSOS is still a relatively obscure title for the public, giving him much less public power than DPM would do. And on the fourth hand it protects him - FSOS suggests more "OK guys, I'm here to keep things organised!", whereas DPM suggests more "I'm the guy you need to stab in the back if you want any power". (DPMs might not become PMs, but only because they get bumped off before they can. It's like being first in line when everyone else in the line has a crossbow. On the one hand, if nobody kills you, you're first in the door. On the other hand, you're standing there with everyone else aiming at your back, and someones' bound to pull the trigger eventually.)
Regarding the second part of the job title: Minister of the Cabinet Office... what's that?
Well, the Cabinet Office is the administrative body that supports the PM and the Cabinet, co-ordinates between ministries, etc. It has become increasingly powerful, as more government policy is centralised away from individual ministers - under Blair, for instance, the CO contained groups like the Strategy Unit and the Policy Unit, which often decided government policy before Cabinet had even seen the proposals. [those units don't officially exist anymore, but equivalent groups do].
Now, Green won't be in charge of the Cabinet Office. That's the Minister for the Civil Service, which is a title that is always held by the Prime Minister. The Lord President of the Council (who is also the Lord Chancellor, the Secretary of State for Justice, and the Leader of the House of Commons) and the Chancellor of the Duchy of Lancaster are both officially ministers of the Cabinet Office too. And then there's the Parliamentary Secretary to the Cabinet Office, who is a junior minister in charge of actually running the Cabinet Office.
Instead, Green is just a "Minister of the Cabinet Office", a non-cabinet position (he'll sit in cabinet as FSOS), which basically means that he'll be doing stuff within the CO. I expect the parliamentary secretary will do the actual running of the department, while Green will have a freeer hand overseeing key policy units and liaising between other ministers.
Note well: although we generally talk of 'ministers', like the 'Justice Minister', the highest cabinet miniters (other than a few old titles like chancellor) are officially "secretary of state for ___" - "minister for ___" is a second-rank title, followed by "parliamentary secretary", followed by "parliamentary undersecretary". These are not to be confused with permanent secretaries, who are the senior civil servants at each department. In particular, the "Cabinet Secretary" is not a minister - he's the senior civil servant, the head of the civil servants in the cabinet office, and he's usually (and currently) the head of all civil servants outside of the foreign office.
As Yes Minister's first episode puts it:
" Sir Humphrey: Well briefly, sir, I am the Permanent Under Secretary of State, known as the Permanent Secretary. Woolley here is your Principal Private Secretary. I too have a Principal Private Secretary and he is the Principal Private Secretary to the Permanent Secretary. Directly responsible to me are ten Deputy Secretaries, 87 Under Secretaries and 219 Assistant Secretaries. Directly responsible to the Principal Private Secretaries are plain Private Secretaries, and the Prime Minister will be appointing two Parliamentary Under-Secretaries and you will be appointing your own Parliamentary Private Secretary.
Hacker: Can they all type?
Sir Humphrey: None of us can type. Mrs Mackay types: she's the secretary."
(to clarify: while a permanent secretary is a civil servant, and a parliamentary secretary or undersecretary is a junior government minister appointed (de facto) by the PM, and a private secretary is a civil servant who is effectively a personal assistant to a minister, a "parliamentary private secretary" is an interesting thing - they're a politician appointed to help out a minister, but they're appointed, at least theoretically, by the minister themselves, rather than by the PM.)
And I hope that everyone is now completely without any sort of lingering oconfusion...
But the river tripped on her by and by, lapping
as though her heart was brook: Why, why, why! Weh, O weh
I'se so silly to be flowing but I no canna stay!