The kayak (or just 'boat') used in whitewater kayaking is different than those used in Sea Kayaking. The boat is typically made of a tough linear-linked plastic that is slightly flexible and very strong. Boats can range in size from barely long enough to hold the paddler, up to 12ft or longer.
There are generally three 'sub-hobbies' in whitewater kayaking: river running, playboating, and creeking.
River Running can be thought of as more of a tour down a river. You're there to enjoy the scenery and some challenge of the whitewater.
Creeking is a pure adrenaline form of kayaking involving very technical and difficult rapids. This is typically in the class IV or V range and can include waterfalls, very small and tight rivers, and/or very large and big volume water.
A great place to learn more about the details of whitewater kayaking is in the whitewater kayaking faq
See also: canoeing, paddling, whitewater