Stories of a short, fat bastard.

The primaries are sort of similar to the way party nominations work in parliamentary systems, where it's people who are technically the party members who vote for the leader - or, in this case, the presidential candidate. It's easier to get "into" the party for this than it is in a parliamentary system; in the US you just go out and register yourself as a voter for either party when you're registering to vote in the presidential election, and that entitles you to vote in the primaries for whichever party you sign up for. Registered REpublicans or Democrats can vote for absolutely whoever they want at the actual "choose the leader" election, but for choosing candidates it's limited to party affiliation.