http://www.yogabasics.com/
A guide to yoga practice

http://kidshealth.org/teen/food_fitness/exercise/yoga.html
Yoga for teens

http://www.holisticonline.com/yoga/hol_yoga_home.htm
Very useful information about yoga - types, benefits, exercises, poses, etc.

http://nccam.nih.gov/health/yoga/
National Center for Complementary Alternative Medicine (NCCAM) tells you about yoga practice.