The mythology of most comic book superheroes is a bizarre, complex mix of returns from the grave, changes in identity and shifting alliances. It's what keeps them entertaining after seventy years of someone fighting the same clown.
The general public, however, often doesn't keep up with their medium of choice as obsessively as comic book fans do. The people behind the X-Men movies weren't privy to that last sentence, and decided that audiences were stable enough to handle a little confusion.
The X-Men movies have never really made it clear whether they're in the same continuity or not. That's up to the unfortunate fans to argue about, and thus, never really decide on.
The X-Men have always been a mixed bag in the Marvel universe. Sometimes, you get iconic heroes like a man who has to wear special glasses to protect those around him from his destructive beams, or a Holocaust survivor who also happens to be one of the most powerful men in the world. And other times you get a little guy with leeches inside of him or Darwin, a victim of constant evolution, and part of a group of X-Men that sucked so bad that they were eventually wiped from history. It's this inconsistency that makes them play second fiddle to guys like Spider-Man and their own teammate, Wolverine.
The first X-Men series was three films, X-Men, X-Men 2: X-Men United, and X-Men: The Last Stand. X-Men is an alright movie. It has some great action set pieces and is a decent introduction to the team. However, it also includes things like Halle Berry saying ?Do you know what happens to a toad when it's struck by lightning? The same thing that happens to everything else.? This is the equivalent of a screenwriter throwing his hands up in the air and saying ?Fine. Fuck it. I don't care enough to come up with a cool comeback. It's just the same thing. That's what it is.?
The second X-Men film is awesome. It takes everything bad about the first film and puts it in a cameo role, and features the awesome Brian Cox as the bad guy. The third X-Men film is a mess in terms of everything, as it falls flat on its face in any attempt to show something dramatic or meaningful.
Charles Xavier, Cyclops AND Jean Grey all die in the same movie, but since The Last Stand is built so that you instinctively don't give a shit about any of it, you'd find a greater narrative punch by asking Hugh Jackman to call his Mom and have him explain to her what he thinks the plot of the movie is. And that doesn't mean that the plot is complicated to the point of confusion, but it lacks all meaning whatsoever. I've read the backs of cereal boxes that felt more like touching stories than X-Men: The Last Stand.
Two prequel movies were planned, both showing the origins of a popular character, Wolverine and Magneto. Only one ever got made, X-Men Origins: Wolverine, though, with a title like Origins, one could expect it to be an entirely new group of films. Luckily, for viewers everywhere, there wasn?t any more of them. X-Men Origins: Wolverine is an experimental art piece, designed to throw a bunch of characters together with the least coherency possible.
The film contradicts certain plot points in the original three X-Men films, but it was never labeled as a part of any other continuity. ?It has Hugh Jackman as Wolverine, so one can assume that it's in the same storyline, as that's what a logical person would assume, but he has interactions with certain people, like Sabretooth, that would lead you to believe otherwise.
But its comics, right? Shit like that happens all the time.
Then, X-Men: First Class came out. At first, it seems like this may be a prequel, but once again, interactions amongst the characters and certain plot points make it clear that it isn't. Is it in the same story as Wolverine? One can only hope not, because that film is Wolverine, and filmmakers would rather make a film in which Magneto literally is Hitler, than associate with that movie again.
There are two more X-Men movies coming out, X-Men: Days of Future Past and another Wolverine movie. The Wolverine film has been reported as both a standalone film and a sequel, though not a sequel to Origins: Wolverine, which would mean that it's not in the original X-Men series, or is it? The new X-Men team film is a sequel to First Class, but will have some time travelling aspects in it, leaving fans to speculate that maybe the cast of First Class will interact with some of the cast of the first series. But is First Class in that continuity? Hugh Jackman plays Wolverine in First Class too, telling Xavier and Magneto to ?Fuck off.?
Oh, well. At least we have Iron Man.