Yoga is a psycho-physical discipline with roots going back about 5,000 years. Hatha Yoga, which includes postures and breathing, and is the form most popular in the West, is actually part of Raja Yoga, the path of self control. Western science has been studying Yoga for nearly 50 years, and the evidence shows numerous physical and psychological benefits from Yoga. A recent study with heart patients showed that those who followed a stress reduction program that included many Yoga practices who exercised or did nothing. Further, preliminary studies in the United States and India suggest that Yoga may be helpful for specific conditions, such as asthma, epilepsy, anxiety, stress and others.
Yoga is a process.
The idea is to focus your awareness on what you are doing and how you feel as you perform the postures. There's also a difference on the physical level. Weight training, for example, makes you stronger by breaking down and rebuilding muscle tissue. It's this breaking down and rebuilding that results in the bulky muscle look. Yoga increases strength by toning the muscles.