Earlier this month, Marvel unveiled some photos from Odin's vault in Thor, and one of the items on display is the Infinity Gauntlet, which Thanos wears. Here's a description of the Thanos character below.
"Debuting in the Bronze Age of comic books, the character has featured in over three decades of Marvel continuity and a self-titled series. The character's name is a derivation of Thanatos, the personification of death and mortality in Greek mythology. One of his more prominent storylines to date-the Infinity Gauntlet series-portrayed him as a mad cosmic tyrant bent on slaying half of all sentiences existing then in Marvel's multiverse in order to appease the Marvel Universe's actual (and more powerful) embodied personification of Death in an attempt to win its love."
It isn't known how large a part Thanos, who is also known as the "Mad Titan," will have in The Avengers, or who will be playing him. Nothing has been confirmed by Marvel Studios regarding Thanos at this time.
Marvel's The Avengers was released May 4th, 2012 and stars Robert Downey Jr., Chris Evans, Mark Ruffalo, Chris Hemsworth, Scarlett Johansson, Jeremy Renner, Tom Hiddleston, Clark Gregg. The film is directed by Joss Whedon.