12 U.S. states have adopted same-sex marriage over the past decade, and many other states have adopted legislation specifically intended to prevent same-sex marriages from being performed or recognized within their borders. The landscape has just changed on that front, though: the 1996 Defense of Marriage Act, which barred federal recognition of same-sex marriages, has been ruled unconstitutional by the U.S. Supreme Court; here's the ruling itself. From the NBC News version of the story:
"The decision was 5-4, written by Justice Anthony Kennedy.
“'DOMA instructs all federal officials, and indeed all persons with whom same-sex couples interact, including their own children, that their marriage is less worthy than the marriages of others,' the ruling said. 'The federal statute is invalid, for no legitimate purpose overcomes the purpose and effect to disparage and to injure those whom the State, by its marriage laws, sought to protect in personhood and dignity.'"
One major area this affects is tax law; that's one of the salient points in U.S. v. Windsor, the case that drove the court's conclusion. There's more on the story at many major news outlets, and at law-centric sources like SCOTUSblog. The Boston Globe is also live blogging various reactions.
Update: 06/26 16:58 GMT by T : In a separate decision, the court disappointed supporters of California's Proposition 8, a law passed by voter initiative, under which "only marriage between a man and a woman is valid or recognized in California." The court ruled that the private parties which had taken up the Prop 8 banner did not have standing to do so; as the story says, "The 5-4 decision avoids, for now, a sweeping conclusion on whether same-sex marriage is a constitutional "equal protection" right that would apply to all states."