Firts of all, I think that people who practice exteme sports are irreverent and anti-establishment. They have their own lifestyle and expresse theirselves through action. That is the key! It's an attitude, a way of life and, for someone, it may be a religion.
There is a lot of kinds of extreme sports.
First exemple:
Earth:
Skateboarding, Motocross, Mountain Biking, Caving, Rock Climbing, Free Climbing, Mountaineering, Zorbing
Water:
Surfing, Waterskiing, Windsurfing, Kayaking, Scuba Diving, , White Water Rafting, Jet Skiing
Snow and Ice:
Snowboarding, snow skiing, Ice Climbing,
Air:
Base Jumping, SkyDiving, Wing Suiting, Bungee Jumping, High-lining, Hang Gliding, Paragliding