The word yoga means "union", referring to combining your mind with the physical body. You are practicing it to calm your mind, by holding a present focus and release any limitations in the body. Yoga can be practiced in different ways, ranging from a physical flow yoga with music to a sedentary breathing exercise in silence. An important factor in practice is the breathing, which allows the body to heal by a more energetic respiration. It's said, according to Patanjali in the Yoga Sutras, the eightfold path are eight steps that basically act as guidelines on how to live a meaningful and purposeful life.
Who can do yoga?
Everyone who can breathe.