The X-Men are a group of mutants under the command of Prof. Charles Xavier who strive to save mankind from the evil forces of Magneto and pretty much anyone else that threatens the world. Some of the more well known X-Men include Wolverine, Cyclops, Emma Frost and Storm.
The Marvel Universe includes many different heroes who on occasion have crossed comics to battle evil and sometimes each other. Some of the biggest clashes have been between Spiderman and the half vampire/half human Blade and the adamantium clawed Wolverine. It seems not everyone is impressed with Spiderman's sarcastic sense of humor.
One of the most intense instances in the Marvel Universe was the death, actually assassination, of the first Avenger, Captain America. This shook up the Marvel Universe and help catapult the brand into new found popularity. In recent years, the comics have been overshadowed by the blockbuster movies about characters such as Spiderman, Iron Man, Thor and The Avengers.
While the movies do depart somewhat from the canon of the comics, they are a good representation of the Marvel Universe. Although, they cannot compare to the sheer volume of information covered by decades of comics. I definitely don't see the comics going away, but I hope Marvel does a better job tying in the comics to their big screen counterparts.