I started reading some variant of The X-Men comic books in the early '80s and stopped sometime in the '90s. To be honest, I've never tried to follow the complex continuity of the various series (though I do remember being very excited when The New Mutants and Excalibur debuted). In IGN's surprisingly easy-to-follow video, we explore a timeline of all the X-Men and -Women, with brief explanations of the various factions and sub-teams they joined.
For me, this is a nostalgic trip and also a serious memory test. Tons of these characters don't register (I probably didn't buy those books, I guess), many call up an "Ohhh, that guy" reaction. There are many "X-Men" who only joined the team for very brief runs.
Note that this video contains some spoilers if you intend to read the last 50+ years of comics and just haven't gotten around to them yet. But if you're a fan or don't care about mild spoilers you'll probably forget anyway, strap yourself into the Blackbird, we're going on a trip back in time:
If you're a superfan, there's a delightful argument in the YouTube comments regarding whether X-Force and X-Factor members should be included here.