A primary election is conducted so that voters can select candidates for a subsequent election (nominating primary). Prior to general elections, they allow for each political party to decide its nominee for the upcoming general election.
Even numbered years, 2012, 2014 etc., are federal general election years, so many states will be holding both presidential preference primaries to select the presidential candidates, and state primaries to select federal congressional candidates.
Both the presidential preference primaries and the state primaries are federal level elections -- so they are open to all eligible overseas citizen and uniformed services voters. U.S. Congressional nominees (U.S. House of Representatives and Senate) are decided in state primary elections which are held by all states. Be sure to consider exercising your ability to vote in the state primary as well as the general (presidential) election.
Please consult the State Voter Information Directory on this website to see if your state is holding a primary election and to confirm the deadlines for registration and ballot request.