The election process begins with primary elections and caucuses. These are two methods that states use to select a potential presidential nominee. Primary elections and caucuses differ in how they are organized and who participates. And rates of participation differ widely.

Primaries
Primaries are run by state and local governments. Voting happens through secret ballot. Some states hold “closed” primaries in which only declared party members can participate. In an open primary, all voters can participate, regardless of their party affiliation or lack of affiliation.
Caucuses
Caucuses are private meetings run by political parties. They are held at the county, district, or precinct level. In most, participants divide themselves into groups according to the candidate they support. At the end, the number of voters in each group determines how many delegates each candidate has won.
When are they held?
State and local governments determine the dates on which primary elections or caucuses are held. These dates, and the amount of time between a primary and general election, significantly influence how early candidates begin campaigning and the choices they make about how and when campaign funds are spent.
In the run-up to presidential elections, victories in primaries held very early in the election year, such as that in New Hampshire, can influence the outcome of later state primaries.