The first kind of Hatha yoga is called Iyenga yoga, which concentrates on alignment and muscle-to-bone relationships. It is fluid and dance-like, and the work is very pose-to- pose, emphasizing grace and simplicity of movement. It is not as physically demanding as some of the other styles, and might be particularly appealing to beginners or the less athletic. Iyenga gives the body tone and flexibility and increases mind-body awareness. It can also reduce stress and induce calm.

Ashtanga yoga is very intense. Much more athletic than the other styles of Hatha yoga, it can be physically demanding. If you're looking to yoga as your main source of fitness activity, this is the route to take. The "power yoga" classes springing up all over the country in health clubs and aerobic studios are hybrids or offshoots of Ashtanga yoga. (Get ready to do lots of push-ups!)

In Ashtanga yoga, you learn a series of postures which you perform repeatedly. There are six series in all, but for the first several years of classes you may just work on the first one. An entire series might take an hour and a quarter to perform. You begin by learning the first few movements and keep adding on new poses as you get better.

Ashtanga builds endurance and strength - runners love it, for example. The fact that you do the same things in each class in the same order creates a growing sense of mastery over the familiar, but difficult, exercises, and makes it easy to chart your progress as you become more proficient.