In order to have elections, you need candidates. The big question is, how do you get the candidates? Well of course anyone who meets the requirements can step up and say I want to be a candidate. Usually, though, candidates that belong to political parties will have an advantage over independent candidates. Most candidates will, therefore, seek the nomination of a party. How do you get a party’s nomination? That is where primaries come in. You win the parties primary election, you’re their candidate.
This wasn’t always the way candidates were nominated and is quite particular to the United States. Until the early 20th century, so called party bosses would select the party’s candidates. During the progressive movement, there was a strong drive to fight corruption. Since party bosses were widely though of as corrupt, the movement sought to make the nomination process more democratic. To put it simply the common party supporters should have more of a say in who the party should nominate. The result was the introduction of primary elections.
Since then parties have been holding primary elections to pick their candidates. Several different forms of primaries have emerged. There is of course the closed primary. Here only voters registered with the party holding the primary are allowed to vote. Another form of primaries are open primaries. These primaries have different degrees of openness. The main goal is to allow independent voters a chance to take part in the primaries. In the most open of the open primaries, anyone can vote. It is, therefore, possible for voters registered with the opposing party to vote in these primaries. Thus parties can try to influence who their opponents candidate will be. If they are successful, they could help a “weaker” candidate gain the parties nomination.
Of course there can be a lot more said about how primaries work and their history. The nitty-gritty of primaries, though, is that they are used up by parties to pick their candidates for the general election. So when you start hearing about primaries, you will know that general elections are coming up.
Here is a map of the states and what form of nomination election they hold (Caucus, Primary).