A spiritual and ascetic discipline, a part of which, including breath control, simple meditation, and the adoption of specific bodily postures, is widely practiced for health and relaxation.
Yoga is much more than those poses. Derived from the Sankrit word "yuj" which means "to unite or integrate"; yoga is a 5000 year old Indian body of knowledge. Yoga is all about harmonizing the body with the mind and breath through the means of various breathing techniques, yoga postures (asanas) and meditation.
Yoga is not just stretching. But while stretching is certainly involved, yoga is really about creating balance in the body through developing both strength and flexibility.
In addition to practicing the poses, yoga classes may also include instruction on breathing, call and response chanting, meditation, or an inspirational reading by the teacher. The variety and amount of this will depend on the individual teacher and the yoga style in which he or she is trained. Typically, a yoga class at a gym will be more focused on the purely physical benefits of yoga, while one at a yoga center may delve more into the spiritual side. Some people find that the physical practice of yoga becomes a gateway into a spiritual exploration, while others just enjoy a wonderful low-impact workout that makes them feel great. Whatever your tendency, you will be able to find a yoga class that suits your style.