From Academic Kids
A primary election is an election in which registered voters in a jurisdiction select a political party's candidate for a later election (nominating primary). Primaries are sometimes open only to registered members of that party, and sometimes open to all voters. In open primaries, voters must typically choose only one primary to participate in that election cycle. Louisiana, U.S.A. is an exception. Until 2004 in the state of Washington, U.S.A., this was also not the case, and voters were able to vote in all parties' primaries on the same ballot, though not for more than one candidate per office. This "blanket primary" was struck down by the United States Supreme Court as violating the First Amendment guarantee of freedom of assembly in the case California Democratic Party v. Jones.
Primaries can also be used in nonpartisan elections to reduce the set of candidates that go on to the general election (qualifying primary). (In the U.S. many city, county and school board elections are nonpartisan.) Generally twice as many candidate pass the primary as can win in the general election, so a single seat election primary would allow the top two primary candidates to participate in the general election following.
Because many Washingtonians were disappointed over the loss of the blanket primary, which the Washington State Grange helped institute in 1935, the Grange filed Initiative 872 in 2004 to establish a qualifying primary for partisan races, thereby allowing voters to once again cross party lines in the primary election. Supporters claimed it would bring back voter choice; opponents said it would exclude third parties and independents from general election ballots, would result in Democrat or Republican-only races in certain districts, and would in fact reduce voter choice. The initiative was put to a public vote in November 2004 and passed.
In elections using voting systems where strategic nomination is a concern, primaries can be very important in preventing "clone" candidates that split their constituency's vote because of their similarities. Primaries allow political parties to select and unite behind one candidate.
In the United States, the small state of New Hampshire draws national attention every four years because it has the first U.S. presidential primary. (In 2004, the Washington, DC primary had the distinction of being the first in the nation; however, it was only binding for the Green Party. The Democratic Party's vote in the primary was non-binding, and only 4 of the 9 Democratic candidates were listed on ballots.)
Types of Primaries
Open. Voters may vote in primaries of a party of their choice, the choice to be made at the voting booth.
Closed. Voters may only vote in a primary if they are registered members of that party.
Semi-open. Voters registered with a party may only vote in the primary of their party. Independents may choose which primary to vote in at the voting booth. Some U.S. states allow parties to determine whether independent voters can vote in their primaries. For example, in West Virginia, Republican primaries are open to independents, but Democratic primaries are closed.
Blanket. No longer in use, allowed voters to vote in either primary, switching party primaries with each office (Ex. Republican Presidential primary, Democratic Gubernatorial Primary, Republican Congressional Primary).
Run-off. If no candidate receives a majority (50%) the top two candidates may face off in a run-off election. This system is used in Louisiana for all state, local, and congressional elections.