THe Oscar nominees and eventual winners are decided by members of the Academy of Motion Picture Arts and Sciences. The process starts in November of each year and continues untll the ceremony takes place in February.
Motion picture studios campaign to get their candidates noticed, rather like campaigning for a politician. In December, the Academy members (there are about 6000 of them) are sent ballot papers, which they use to nominate possible winners in their own expert categories - in other words, actors who are Academy members will decide which actors to nominate, directors will choose directors and so on. The ballot papers have to be sent back in January.
When the nominees have been chosen, the members are balloted again, this time to make a single selection for each category. In this ballot, they can vote across all categories in most cases, not just their own.
The results are kept secret until the night of the Oscar ceremony.