1. The primaries
The primary is the first step in choosing a party's candidate. In most countries, the party picks the candidate. But in the US, voters who declare support for one party or another get to choose from a list.
US presidential elections are always held in November. State primaries begin the preceding January, with candidates campaigning against other members of the same party for the nomination.
Delegates at the party's national convention in the summer make the final selection. In the primaries, voters select these delegates, who in most cases have pledged to support a particular candidate. Candidates need a majority of delegates at the convention to win.
Some states, such as Iowa, use a caucus system rather than primaries to choose their delegates. Whereas in primaries people simply indicate at the ballot box which delegates they support, caucuses are more complex and work by selecting delegates through a number of stages.