The Academy of Motion Picture Arts & Sciences consists of 5,830 voting members in 16 branches. Of those, the largest group is the Acting branch which consists of 1,251 members.
Among other things, this structure means that films with strong acting in them have better chances to get the Best Picture nomination. Prime examples this year are “Babel,” “The Departed,” and “Little Miss Sunshine,” which boast large and impressive ensembles, and “The Queen,” dominated by Helen Mirren's performance.
Each branch nominates in its own category: Actors votes for actors, director for directors, and writers for writers.
Special voting groups within the Academy pick the nominees in other categories (documentary, foreign-language films, makeup, short films, sound editing, visual effects), with all the voting members nominating best pic.