If a presidential candidate wins a state primary election, what exactly do they get?

I don't know a lot about politics, but I want to learn. For example: If Republican Presidential candidate "John Hancock" wins the Virginia State primary in February 2008, what happens to the other republican candidates? Will they still be on the ballot for the general election in November? Can you lose a state primary and still be on the ballot in that same state for the general election? If so, then what exactly is the primary election for? What does the winner get ?

Public Comments

1. A trophy, "Don't Mean Nothin' Memorial Trophy", it's a fairly prestigious award really.
2. They get momentum in a campaign and attention from the media. They show that they may be for real. They may get those delegate votes at their party's convention. For instance, a candidate wins in California, they win 55 delegate votes for president at the convention. Delegates can vote for whom they choose, but they typically go the way of their state.
3. The winner of primary elections for President get "delegates" who are committed (at least on the first ballot) to vote for that candidate at the party's convention. The candidate who gets the majority of the delegate votes wins the party's "nomination".
4. Each state's primary is important, but the way it has become, the earliest primaries are the most important, because voters almost always follow the path of the first three or four states' primaries (Iowa, N. Hampshire, S. Carolina). If "Hancock" wins Virginia's primary, he wins the points from that state. Sort of like how the real Presidential Election is. However, it will most likely be the early states, even though they aren't the largest, which will determine who represents their party for the general elections. There will only be one candidate from each party on the general election ballots. That person is the winner of the combined primaries.
5. Party Nomination. With enough states won, they will be officially named the nominee of the party and move on to the general election, whereas all others are no longer running aside them on the ballot.
6. arterex has it right. The delegates are committed for the first vote only. And the other guy has one good point; the big MO. Momentum from looking like a winner rather than a wannabe.
7. The primary is held to determine which candidate is designated to run, as a member of her/his party, in the general election. In the general election, both party's selected candidates run against each other for the contested office,