SXSW is a difficult creature to try to describe to someone. Sure, one could just say: "It's a music festival that's in Austin, Texas." But that fails to describe many of the nuances that are present at the festival.
For instance, the festival has expanded to more than just music. It has now incorporated tech, film, comedy, and video games under it's umbrella. True, all of them are segmented into their own "mini-festivals", but they overlap with one another over nine days, making a combination of artists, business people, film critics, gamers, and general festival goers into a large mass.