In the US, there are only two parties considered by most voters - the Democrats (the liberal party) and the Republicans (the conservative party).
Others, like the Libertarian, Green and Independent parties occasionally put forth a nominee.
At present, presidential hopefuls are battling for their party's nomination in caucuses and primary elections across the country.
State governments run primary elections the same way as a general election.
If a candidate wins a primary election, they win either all or a proportion of the state's delegates. These delegates will then vote for them at the party convention, where the presidential nominee is named.
A handful of states have caucuses instead of primaries. The parties run caucuses in precincts across the state.
Caucuses give parties more flexibility in determining the rules. In Democratic caucuses, votes are determined by standing in groups around a room.