The word yoga comes from the Sanskrit for "union" or "yoking," which is apt, since this ancient Indian discipline is both a physical and spiritual activity. For most of yoga's history, the emphasis was on spiritual enlightenment, which called for little more than the classic cross-legged meditation pose. But today's Americanized versions focus on physical benefits. This has led to increasing numbers of injuries, especially among overzealous beginners.
Most American versions come out of hatha yoga, which is the practice of postures, or asanas, held for several minutes if you're an expert, or only a few seconds if you're just beginning. These positions stretch, strengthen, and relax specific muscle groups. You need to learn proper techniques of breathing and concentration to be able to hold the postures properly. Some asanas are simple, merely stretching and aligning the body, while your breath and heart rate are kept slow and steady. Others are complicated, difficult to master, and very strenuous. Some involve twisting the spine; others require standing on your head or shoulders. Many of these are only for advanced practitioners; they can injure a person who is not flexible or strong enough or has physical problems.