Uncanny X-Men are a group of planet wide mutant police, led by Storm with Nightcrawler, Sage, Bishop, Racheal Summers, Cannonball and Wolverine on the team. Written by Chris Claremont, if you previously enjoyed his stuff, check it out, if you didn't, I'd stay away, or only check it out occasionally.
X-Men are a group of mutant adventurer's. Havok, Polaris, Juggernaut, Wolverine, Gambit, Rogue, Xorn and Iceman are on the team. They seem to go after and battle any mutant terrorist cells, and try to diffuse armed situations. They have a touch of super-heroism and maybe a smattering of black ops, but unlike the Uncanny title, their team doesn't have any sort of official sanction for world police action, they're mutants taking care of mutants, or mutants taking care of regular joes when bad mutants attack. Written by Chuck Austen, so it gets soap opera-y and often times convoluted as heck. Check it out if you want to, but his barrels of negative reviews stave off most people. New writer Peter Milligan coming on board soon, he did great stuff with X-Statix, so hopefully he can smoothe out this one.
Last but not least. Astonishing X-Men. A group of mutant superheroes who want to go out and help public relations by just going out into the world and saving people. Avengers--but with a prerequisite of an X-gene. Members are Cyclops, Wolverine, Emma Frost, Beast, Shadowcat and Lockheed. With another person introduced, but unsure whether he'll be sticking around. Written by Joss Whedon, and recieving rave reviews, this is a great title, and I'd suggest you pick this up if you want to get back into X-Men comics. It's only going into issue 5, so you can scrounge around and start from the very beginning.