The Primary (California)
In states holding them, presidential primary elections are open to all registered voters. Just like in general elections, voting is done through a secret ballot. Voters may choose from among all registered candidates and write ins are counted. There are two types of primaries, closed and open. In a closed primary, voters may vote only in the primary of the political party in which they registered. For example, a voter who registered as a Republican can only vote in the Republican primary. In an open primary, registered voters can vote in the primary of either party, but are allowed to vote in only one primary. Most states hold closed primaries.

Primary elections also vary in what names appear on their ballots. Most states hold presidential preference primaries, in which the actual presidential candidates' names appear on the ballot. In other states, only the names of convention delegates appear on the ballot. Delegates may state their support for a candidate or declare themselves to be uncommitted.

In some states, delegates are bound, or "pledged" to vote for the primary winner in voting at the national convention. In other states some or all delegates are "unpledged," and free to vote for any candidate they wish at the convention.

The Caucus (not California)
Caucuses are simply meetings, open to all registered voters of the party, at which delegates to the party's national convention are selected. When the caucus begins, the voters in attendance divide themselves into groups according to the candidate they support. The undecided voters congregate into their own group and prepare to be "courted" by supporters of other candidates.

Voters in each group are then invited to give speeches supporting their candidate and trying to persuade others to join their group. At the end of the caucus, party organizers count the voters in each candidate's group and calculate how many delegates to the county convention each candidate has won.

As in the primaries, the caucus process can produce both pledged and unpledged convention delegates, depending on the party rules of the various states.

Source: About.com About the Primary - Caucus - Convention System

History Behind California's Primary Election System

Source: California Secretary of State

Closed Primary System
A "closed" primary system governed California's primary elections until 1996. In a closed primary, only persons who are registered members of a political party may vote the ballot of that political party.

Open Primary System
The provisions of the "closed" primary system were amended by the adoption of Proposition 198, an initiative statute approved by the voters at the March 26, 1996 primary election. Proposition 198 changed the closed primary system to what is known as a "blanket" or "open" primary, in which all registered voters may vote for any candidate, regardless of political affiliation and without a declaration of political faith or allegiance.

On June 26, 2000, the United States Supreme Court issued a decision in California Democratic Party, et. al. v. Jones, stating that California's "open" primary system, established by Proposition 198, was unconstitutional because it violated a political party's First Amendment right of association. Therefore, the Supreme Court overturned Proposition 198.

Modified Closed Primary System
California currently has a "modified" closed primary system. SB 28 (Ch. 898, Stats. 2000), relating to primary elections, was chaptered on September 29, 2000 and took effect on January 1, 2001. SB 28 implemented a "modified" closed primary system that permits unaffiliated ("decline to state") voters to participate in a primary election if authorized by an individual party's rules and duly noticed by the Secretary of State.
(Ch. 898, Stats. 2000)

Deadlines for Political Parties that have Adopted Party Rules in Accordance with SB 28
(Ch. 898, Stats. 2000) for the February 5, 2008 Presidential Primary Election  

Political parties have until 135 days prior to an election to provide a written notice to the Secretary of State indicating the adoption of a rule allowing decline to state voters to vote the ballot of that individual party. The deadline for the February 5, 2008 Presidential Primary Election was September 23, 2007.

List of Political Parties that have Adopted Party Rules in Accordance with SB 28 for the Presidential and Statewide Direct Primary Elections

The following qualified political parties filed the requisite statement notifying the Secretary of State that they adopted a party rule permitting unaffiliated voters to request their party ballot in the February 5, 2008 Presidential Primary Election to participate in the primary election for partisan offices:

Democratic Party, letter dated September 19, 2007, American Independent Party, letter dated August 23, 2007