A federal appeals court ruled Tuesday that California's ban on gay marriage is unconstitutional. The U.S. Ninth Circuit Court of Appeals ruled that Proposition 8, the 2008 ballot measure that limited marriage in California to one man and one woman, violated the equal protection rights of gays and lesbians. Now, a forthcoming appeal could pave the way for the U.S. Supreme Court to rule on gay marriage as early as next year. Here's what you should know about the "landmark" decision:

Remind me: What exactly is Proposition 8?
In 2008, a California state court ruled that an existing ban on gay marriage was unconstitutional, allowing same-sex couples to legally marry. Almost immediately, groups that opposed gay marriage began circulating petitions that would eventually put Proposition 8 on that fall's election day ballot. Prop. 8, a proposed amendment to the California Constitution decreeing that marriage was an institution reserved for one man and one woman, was supported that November by 52 percent of California voters. In the brief window between the court ruling and Prop 8's ballot success, 18,000 same-sex couples legally married in California.

Why is the issue in court again?
Judge Vaughn R. Walker, a federal judge in San Francisco, struck down Prop. 8 in 2010, declaring the amendment unconstitutional because it violated the equal protection rights of gay couples. Opponents of same-sex marriage appealed the decision, says Howard Mintz at the San Jose Mercury News, on the grounds that "there is a state interest in preserving the traditional definition of marriage, particularly the importance of procreation in heterosexual marriage." Tuesday's ruling was the result of that appeal.

What exactly did the appeals court rule?
A three-judge panel ruled 2-1 to uphold Walker's decision. "Proposition 8 served no purpose, and had no effect, other than to lessen the status and human dignity of gays and lesbians in California," the court said. The Constitution "requires that there be at least a legitimate reason for the passage of a law that treats different classes of people differently. There was no such reason that Proposition 8 could have been enacted."

Will this affect other states?
Not necessarily. Tuesday's decision is not intended to be applied broadly, the court said, and was crafted to apply only to California. The peculiar circumstances of what happened in California — "a right to same-sex marriage withdrawn by a vote of the public" — is what ultimately led to the ruling, says Jeffrey Toobin at CNN, and should have no effect on other states currently weighing the issue.

Can same-sex couples get married in California now?
Not yet. The appeals court said that gay marriages cannot resume until Prop. 8's supporters have the opportunity to appeal to a larger panel of the Ninth Circuit. If and when the appeal is filed, which it inevitably will be, says Peter Henderson and Dan Levine at Reuters, gay marriage will be kept on hold "pending future proceedings." Gay marriage opponents have another option, too, says Adam Nagourney at The New York Times. They can try to take the issue directly to the U.S. Supreme Court. However, says Toobin, the Supreme Court might opt out of weighing in. And even if the nation's highest court does hear the case, it likely won't be until after the election.

How would the Supreme Court rule?
It's anyone's guess. Some gay activists are apprehensive about the case going all the way to the Supreme Court, says Nagourney. They're "fearful that conservative justices could... codify a ban against same-sex marriage." In the end, the Roberts Court will probably be divided, says Maura Dolan at the Los Angeles Times, "and many legal scholars believe Justice Anthony Kennedy will be the deciding vote." Kennedy is a Republican-appointed justice, says Henderson and Levine, but he has written several important pro-gay rights decisions — though he "has not explicitly endorsed gay marriage."

Sources: AP, CNN, Huff. Post, LA Times, NY Times, San Jose Mercury News, Reuters