BBC also has a good Q&A here: http://www.bbc.co.uk/news/world-us-canada-21943292

Today the court is hearing the case about Prop 8 in California. Tomorrow they will be hearing the case about DOMA, a 1996 law blocking federal recognition of gay marriages from a state allowing it. DOMA for example blocks a state recognized gay couple from filing as married on their federal taxes.