It's a 3 day event at the polo fields.  You can buy tickets to all 3 days, or just single days.  There are 2 large outdoor stages, and 3 huge tent stages.  Bands are playing constantly.  Sets are usually between 30-40 minutes, and set times are staggered between that stage, so even if a couple bands overlap set times you can usually catch half of each.  There's tons of large-scale art installations, and tons of awesome people.
Here are some photos of last year's:
http://jenniegreg.com/Gallery/main.p...+Day+1/&var2=0
http://jenniegreg.com/Gallery/main.p...+Day+2/&var2=0
http://jenniegreg.com/Gallery/main.p...+Day+3/&var2=0