The Supreme Court has ruled The Defense of Marriage Act is unconstitutional by a 5-4 vote.It was struck down on an equal protection principle.  That means if marriage is legal in the state you reside in, you cannot be treated differently under federal law.  In the opinion, the ruling is restricted only in states where same-sex marriages are legal.

DOMA is unconstitutional as a deprivation of the equal liberty of persons that is protected by the Fifth Amendment.

The court ruled in favor of states' rights, saying:

New York’s actions were a proper exercise of its sovereign authority. They reflect both the community’s considered perspective on the historical roots of the institution of marriage and its evolving understanding of the meaning of equality.

That means the Court will not force other states to legalize gay marriage, respecting the states' rights to decide what marriage is, not the federal government.

Proposition 8 case was thrown out since the group bringing the case to the Supreme Court had no standing.  In the 5-4 ruling, it effectively makes same sex marriage legal in California.

The Court does not question California’s sovereign right to maintain an initiative process, or the right of initiative proponents to defend their initiatives in California courts. But standing in federal court is a question of federal law, not state law. No matter its reasons, the fact that a State thinks a private party should have standing to seek relief for a generalized grievance cannot override this Court’s settled law to the contrary.

The rulings have no major widespread effect on the nation's same sex marriage legality.  Essentially, the Court ruled that it is not the federal government's role to decide on the legality of same sex marriage.