The purpose of a primary election is to narrow the field of candidates before a general election. Primary elections allow the people to play a larger role in the nominations of the party. The two main types of primary elections in the United States are open and closed elections.
In a closed primary election, only voters who are registered to a specific political party are eligible to vote. If it is a Democratic primary election, only registered Democrats can vote. In an open primary election, all registered voters can vote, regardless of what political party they are registered with. While in many countries candidates are selected by party leaders, the United States uses the primary election to allow the people to select candidates.