Best Director is one of the original Oscar categories, though in its first year AMPAS recognized directors of dramatic films and directors of comedic films seperately. Ever since that first year, however, there has been no distinction between directors of comedy and drama. John Ford currently holds the record for most Director Oscars won with 4 (out of only 5 nominations - seems like he should have had more).
This year's nominees:
Kathryn Bigelow, The Hurt Locker: So, you may not have heard but Bigelow and James Cameron used to be married. I really dislike how the overriding narrative of this year's Oscars has become "Battle of the Exes." This is the first Oscar nomination for Bigelow, who won the DGA, the Critics Choice, and a whole swath of critics awards.
James Cameron, Avatar: This is Cameron's second nomination as Director. He won previously for Titanic. He won the Golden Globe for his work in Avatar and was nominated for the DGA and Critics Choice.
Lee Daniels, Precious: This is the first nomination for Daniels, who was also nominated for the Golden Globe, DGA and Critics Choice Awards.
Jason Reitman, Up In The Air: This is Reitman's second nomination as Best Director, the first coming for Juno. He also received Golden Globe, DGA, and Critics Choice nominations and is also nominated for Best Adapted Screenplay.
Quentin Tarantino, Inglorious Basterds: This is the second Best Director nomination for Tarantino, the first coming for Pulp Fiction. He is also nominated in the Original Screenplay category. He has received Golden Globe, DGA and Critics Choice nominations for Inglorious Basterds and won the Phoenix Film Critics Award.