In 22 of the 23 categories, plurality determines the winner. However, since 2009, that is no longer the case for the Best Picture award.
This is instead decided by instant runoff voting, which has voters rank candidates in order of preference. Ballots are initially counted for each voter's top choice. If a candidate has more than half of the first choice votes, they win. If not, then the candidate with the fewest votes is eliminated, and the voters who selected the defeated candidate as a first choice then have their votes added to the total of the next choice.
This process continues until a candidate has more than half of the votes.