Twelve officials vote: the seven governors of the Federal Reserve, the president of the New York Fed and four other regional presidents in rotation. The decision comes out at 2 pm New York time, followed by a press conference with the chair.
Traders usually know the likely decision in advance, so the reaction often comes from the wording of the statement, the chair's answers and the votes of officials who disagree. Four times a year the committee also publishes its own forecasts for rates.