Just like every other state, California has historically defined marriage as an institution between a man and a woman. It was not an explicit definition under California law until 1977 but there was never a question in the minds of the people or in the courts before that time.
1977 - California's legislature was explicit and defined marriage as being between a man and a woman. (See Family Code section 300)
2000 - in response to the possibility of other states allowing marriage between same-sex couples, 61% of California voters chose to codify a state law that defined marriage as only being recognized if it is between a man and a woman (to stop out-of-staters from moving into the state as a married gay couple). Refer to Proposition 22, codified in California's Family Code section 308.5.
Despite this clear intent expressed by the people, California's legislature tried twice, both in 2005 and 2007, to allow marriage to extend to homosexual couples. Although the legislature had power to overturn section 300, the Governor vetoed the bill. In contrast, the legislature could not overturn section 308.5 (Proposition 22) without the people's vote, and the courts held that section 308.5 clearly preserved marriage as only being between a man and a woman in California. Thus, the legislature failed in these attempts.
In the last several years, the same substantive rights that are given to married couples have been extended to "domestic partnerships" formed between homosexual partners.
There is no difference in what benefits and responsibilities a domestic partnership receives as a "married" couple, except for the title of marriage (Family Code section 297.5, etc).
On August 12, 2004, the California Supreme Court held that several gay marriages condoned by San Francisco city officials were void and of no legal effect.
On May 15, 2008 in a court case entitled "In Re Marriage Cases," which was really six different cases taken on appeal, four California Supreme Court justices overturned the will of 61% of California voters by holding that the "dignity" of holding the title of marriage is itself a right, that gays seeking marriage were a "suspect class" similar to gender and religion, that sections 300 and 308.5 were in conflict with the California constitution's Equal Protection clause, and that the laws did not meet the standard required to survive that conflict (e.g. they held it was not necessary to serve a compelling state purpose).
Proposition 8 seeks to amend the California Constitution itself to define marriage as only being between a man and a woman. The constitution is the supreme legal document of California, with much more legal clout than a mere statute. This would return the power to the people of California and make it extremely difficult for the courts to overturn this definition in the future without an express vote by the people.