For Oscar nominations, active and life members of the Academy of Motion Picture Arts and Sciences vote by secret ballot upon a pool of submissions in various categories. The submissions that receive the most votes become nominees.
Submissions to every category are contingent upon a plethora of eligibility stipulations the Academy publishes on its website. For example, the film's producer or distributor must sign an "Official Screen Credits" form to corroborate the legitimacy of potential nominees. Each category has its own additional set of specific stipulations by which a possible nominee may be deemed eligible or not. A feature film must be over 40 minutes and exhibited publicly by means of 35mm or 70mm film, for example. Essentially, those who meet the eligibility criteria submit potential nominees, and their peers then cast votes to, first, nominate and then to select the winner.
With the exception of submissions to the Best Picture category, five submissions with the most votes proceed to the nominations stage and final voting. Members of the Academy are allowed, per the Academy's rules, to vote for up to 10 Best Picture submissions. The Academy's voting rules allow colleagues in the same branch to make nominations, so actors nominate actors, and directors nominate directors. The Foreign Language Film and the Animated Feature Film categories are exceptions. These nominations are made by a special committee of invited voters culled from the Academy's existing list of active and life members, with a chairperson appointed by the Academy president.