The decision of the Supreme Court of the United States (SCOTUS) in United States v. Windsor strikes down the 1996 federal Defense of Marriage Act (DOMA). DOMA applied to the federal government and prohibited it from recognizing same-sex marriages -- even in states that had legalized it.
The decision in Hollingsworth et al. v. Perry et al. has the effect of killing California's Proposition 8, which had amended the California State Constitution to define marriage as a union only between a man and a woman. The state officials would not defend this initiative/amendment in court, so the original petitioners chose to do so. The SCOTUS ruled that these petitioners did not have the legal standing to defend Prop 8's constitutionality. Therefore, the former ruling that Proposition 8 is unconstitutional stands without the SCOTUS actually ruling on that matter directly.