This year, there are three types you should know about.
Initiatives are proposed laws or statutes placed on the ballot by citizens. They require a certain number of voter signatures to qualify and, if passed, can be changed by the legislature (with some restrictions).
These propositions aim to change California's constitution. They can be placed on the ballot by the legislature or through the initiative process with more signatures required. If passed, they can only be changed by another ballot measure.
Bond measures ask voters to approve the state taking on debt to fund specific projects, usually related to infrastructure or public works. They're typically placed on the ballot by the legislature.