In that forum of yore, someone asked a bunch of questions about Half-Life 2. I answered them in this exact manner, as there were a couple of posts before mine and I felt a reminder would be useful. There were also a few follow-up posts that went slightly off-topic which I'm including as well. Bear in mind that this was written before Episode 2 came out, and there was still some speculation as to what would appear in it. I don't know if there is any in this Q&A session, but just in case, I felt it necessary to warn you.
Half-Life 2 didn't have an ending. It just stopped.
You asked questions. Let's answer them as best as is humanly possible, and then let's insult everyone else for saying anything different. Y'know, like everyone always does on the interweb. Onward!
"What the hell are the Combine?" - For this, I shall summarize the blurb found upon Wikipedia's rather excellent page upon the subject. Sadly, it proves rock_nog slightly wrong, but I'm sure he doesn't mind. Not that I care.
The Combine is a vast empire spanning multiple parallel universes, which expands its empire by conquering and enslaving populated worlds. By genetically altering the most intelligent races, the Combine (Pronounced COM-bine, with the emphasis on the first part of the word) create advanced soldiers adapted for individual worlds, allowing for the easy destruction of rebellion factions.
Prior to conquering Earth, the Combine had previously tried to enslave the as-yet un-named race which human beings refer to as the Xen race. This race were defeated by the Combine on an entirely different world to Xen, but managed to flee to their final retreat - a dimensional transit bottleneck, or an area of continual contention. This area, which subsequently 'bled' into our dimension, was known as Xen.
Following the death of the supreme being of this race, known as Nihilanth, at the hands of Gordon Freeman, the Combine were able to use the massive portal storm to invade Earth. This resulted in what has since come to be known as the Seven Hour War. Dr Wallace Breen, the former administrator of the Black Mesa Research Facility, was able to negotiate a surrender on Earth's behalf, and as a result was appointed administrator of the Combine's forces on Earth – also known as the Overwatch.
"What the hell is going on in the background?" - I'm not sure what you mean by this one. If you mean the background of the game, then essentially Gordon Freeman, having 'arrived' in City 17 and subsequently going on to destroy countless Combine forces, inspires many of the City's remaining inhabitants to rise up against their oppressors, while the Combine try to track Freeman down and kill him.
"Who are the benefactors? Is this some implied other-dimensional stupid ass [s***]? Or Xenfags?" - The benefactors are the Combine. That's just what Dr Breen calls them. Interestingly, he only properly refers to them as 'The Combine' once, near the end of the game. There are a couple of theories behind this - either that's what he really thinks of them and can't be bothered to hide his feelings anymore, or it's just a slip of the tongue caused by the heat of the moment.
"Alright. So I didn't play ep1-3 or lost coast, but I might. Should I? Would it provide any additional story info? Is a sequel planned?" - Yes, you should. Just remember what they are - short. Lost Coast gives you nothing plot-wise, it's just a setting they cut out during development and then subsequently felt would be a good place to try out HDR lighting while giving fans a bit more HL2. Episodes I, II and III, once finished (only Episode I is currently out. Episode II's release date has slipped to around the end of the year. Don't even ask about Episode III), should reveal a whole lot more plot.
As for sequels, well. These HL2 Episodes as, essentially, Half-Life 3. Gabe Newell, uber-god regarding all things Half-Life, has admitted that a more correct title for these episodes should have been 'Half-Life 3: Episode One', etc, but whatever. A proper Half-Life 3 does seem likely, however nothing's been confirmed. There will be an Episode IV, but it'll be developed outside of Valve and will start a new story arc, so if there's going to be a proper sequel, chances are Valve will start work on it after Half-Life 2 Episode III - Not inspired by Star Wars.
"Not actually a question, but what the hell is up with that ending?" - It's not an ending. Half-Life 2 didn't have one. It just stopped. If you're referring to the boss, well... it's not really an end boss, is it?
If you play the original Max Payne, I think that's the perfect end-boss. Yes, there's a ruddy great chopper you have to take down. But do you kill it with continuous gunfire? No, you [dislodge an aerial mast and] swat it down. Challenging, but logical. Me likey. But in Half-Life 2, you don't really get a boss. A part of me actually likes that. 'This is the way the world ends, not with a bang but a whimper', as is said. You just have to stop Breen, which you do. Right?
As for Ep1, I've never liked destroying Striders or Gunships, because they fall back into the old 'shoot them until they fall down' category. Controlling the rockets is always fun (except when they keep getting shot down, grr), but it just feels sort-of routine. But that's just me, I guess.
The next episodes should be on a more epic scale. Ep1 was dealing with the fallout, Ep2 is escaping to the countryside and Ep3... well, I've no idea, but it's bound to be interesting. We're moving away from City17, so at the very least we'll get different locations, which'll be good.
No-one knows who the G-Man is. That's probably not even his name. There are theories, but nothing's been confirmed. I like the one that he's a member of an incredibly ancient race, older then the Combine, who seek their destruction, and who realizes that Gordon is the one who can do that. One thing is for certain - he is NOT Gordon from the future. Totally.
[As for where the name came from...] It's the filename of the model - gman.mdl - which is where the name came from. Some people thought it was short for 'Government Man', but the events of HL2 kinda disprove that. Truth is, we just don't know. And honestly, if we do find out, it'll spoil the magic. So I don't ever want to.