Those defending California's same-sex marriage ban argued that since California already gave same-sex couples some benefits, preventing them from getting legally married was not discrimination. The 4-3 decision agreed with gay rights groups who argued that calling same-sex unions something other than marriage is akin to second-class status, and is not permitted by the state's constitution. The state has one month to begin allowing marriages to proceed.
Republican governor Arnold Schwarzenegger said he would uphold the court's ruling, and will also oppose a ballot initiative to change the state's constitution in a way that would make gay marriage illegal again. The ballot initiative will likely get enough signatures to appear on the ballot this November for all Californians to vote on it.
My guess is that the ballot initiative will fail, because a similar initiative in 2006 failed in the state of Arizona, which is culturally similar but on balance more conservative than California.
California is the most populous state in the United States with about twelve percent of the United States population. Cities in California are magnets for GLBT people from around the country who are seeking a more tolerant cultural environment, so I would guess that something around 1 in 5 American same-sex couples live in the state and will be allowed to marry because of the California court decision.