New York Times:

The Supreme Court announced on Friday that it would enter the national debate over same-sex marriage, agreeing to hear a pair of cases challenging state and federal laws that define marriage to include only unions of a man and a woman.

One of the cases, from California, could establish or reject a constitutional right to same-sex marriage. Another case, from New York, challenges a federal law that requires the federal government to deny benefits to gay and lesbian couples married in states that allow such unions

In other words, these are the challenges to Prop 8 and DOMA. The ACLU breaks down what's at stake in the two cases over here.