Freestyle is theoretically just playing catch with a friend. But when you see a true freestyler, you'll know the difference, they can do things with the disc you couldn't even have imagined being possible. Check out the sites listed below for access to video clips. Once you've seen how amazing it can look, check out the rules from the WFDF
The World Championships are in Seattle this year, at the end of July, check the website for details.
Here's a couple of Freestyle links: The Freestyle Frisbee Page and FPA - Freestyle Players Association.