California has had two Constitutions. The first Constitution of California was ratified on November 13, 1849, prior to California's admittance into the Union in 1850. California's current constitution was ratified on May 7, 1879 and has been amended over 480 times.
The constitution of California can be amended or revised through:
(1) Legislative proposal. This method requires a roll call vote in the Legislature with two-thirds of the members of each house concurring;
(2) Convention to revise constitution. To convene a constitutional convention also requires a roll call vote in the Legislature with two-thirds of each house's membership concurring. Any amendment proposed by the convention may then be submitted to the voters at a general election; or
(3) Initiative. The electors may amend the Constitution by initiative.
For a summary of the Initiative process see How to Qualify an Initiative on the Secretary of State web site. There is also a link to the Statewide Initiative Guide for step-by-step information on the statewide initiative process.
A majority of electors must vote to approve the proposed amendments or revisions. If successful, the changes will take effect the fifth day after the Secretary of State files the statement of the vote for the election unless the language of the amendment/revision states otherwise. If conflicting measures are approved at the same election, then the provisions of the measure receiving the highest affirmative vote shall prevail. Cal. Const. art. XVIII, §§ 1-4.
The initiative is the process that allows the electors to propose legislation and compel the legislature or the full electorate to vote on the measure. A referendum is the power of the electors to approve or reject all or parts of certain types of statutes. Both the initiative and the referendum process are methods of direct democracy. Propositions are the proposed legislation for either the initiative or the referendum.
To reiterate, as excerpted from Legal Research in California, 5th ed., Call Number: State 1st Floor KFC74 .H36 2004:
"the people of California may directly add, repeal, or amend provisions of the California Constitution or statutes," through ballot propositions. The ballot propositions "are proposed either by the Legislature or by citizens." "Propositions directly from the people are proposed by petitions circulated for the [required] number of voter signatures. Propositions from the Legislature are adopted like other legislative measures." Propositions "propose either bond measures, constitutional amendments, or statutes that change existing statutes previously approved by initiative." Referenda are "propositions that allow the people to approve or reject legislative enactments." Initiatives are "propositions that propose legislative or constitutional changes." pg. 163.
Three informational fact sheets (1) What Are Ballot Propositions, Initiatives, and Referendums; (2) The History of Initiative and Referendum in the United States, and (3) State by State List of Initiative and Referendum Provisions are available through the Initiative and Referendum Institute, a non-profit educational organization that provides explanation of the initiative and referendum. The web site, hosted by the University of Southern California has California specific information on the initiative, popular referendum and legislative referendum.