The dates for GenCon aren't the same, but it's always a Thurs-Sunday.
In short, it's about 20,000 gamers converging on Indianapolis for a few days. Huge HUGE vendor hall of which WotC always has the largest area with absolutely ever gaming publisher represented. Lots of panels about gaming, run by different publishers (GenCon is where WotC makes it's announcements for the most part- 4e was announced here last year).
Also host of the ENnies-- one of the big awards for the gaming industry.