OK, recalling what I learned at university, here, which was some years ago...
There is an American election in November. First, though, the two main political parties have to decide who their candidate for the elections will be. They do this through a series of primaries and state caucuses.
These are both basically state elections where members of each political party in that state vote for the person they want to represent the party in the elections. The main difference between a primary and a caucus is the way they vote,. In primaries, party members go directly to a public voting booth and place their individual vote, much like in a general election. In caucuses, groups of party members meet at a local level and vote individually for their preferred candidate. Then they tot up the votes within their local group, and the 'winner' within the group is the candidate they vote for ( as a group) - ie. they present one nomination asa a group (sorry for convoluted way of explaining it!) Whether a particular state uses primaries or caucuses is largely to do with tradition and state legislation, I think.
The early primaries/caucuses (New Hampshire/Iowa) are traditionally seen as a good indicator of who is going to win (Democrats: Hilary Clinton did well in the NH primary - she was widely expected to lose out to Barak Obama. Republicans: John McCain won the NH primary, as expected). On Feb 5th, 20 states hold primaries, which is why it is called 'Super Tuesday'. It's a big indicator of who'll win for each party.
The candidate for each party is announced officially over the summer at The National Convention for each party (I think?). Then the election campaigning begins, leading up until November.
That's it in a nutshell, methinks.