Yoga originates from the Indian culture as a form of discipline and personal meditation. Though many branches of yoga are practiced in the world, the most common use of the exercise is for relaxation while improving health and fitness.
Yoga is a series of body poses and postures focusing on creating balance in the body through developing both strength and flexibility. The practice can give psychological benefits such as stress reduction and a sense of well being.
The poses can be done slowly to increase stamina or more quickly in succession, creating heat in the body through movement. Although poses themselves will not change, your relationship with them will as you become more familiar with the exercise.
Even if you do not feel flexible or strong enough, your endurance will develop with time; so do not exert yourself on your first time out. The important thing to remember is to work at your own pace with slow and controlled breathing.