Freestyle also is a kind of windsurfing. Might be described as acrobatics on a windsurfing board. Consists of tricks and manoeuvres called moves, such as jumps, spins, rail rides, manipulations with the sail, and so on.
The videos of freestyle moves can be found on the web, e.g. http://www.the-daily-dose.com/moves/moves.htm
Freestyle usually requires flat water and less wind than other windsurfing disciplines.
Freestyle became an official PWA world tour discipline of windsurfing after a successful "King of the Lake" event in Italy at lake Garda in 1998, which has become yearly.