What is Yoga?
What yoga is is a good question. Because yoga is more than just physical exercise. Yoga is a philosophy (not a belief) that consists of several facets (yoga sutras), focused on body and mind. The yoga sutras provide a guideline for conscious and healthy living.
Many people start yoga for the physical movement (Sutra 3: Asana), but soon also experience the mental benefits. By conscious breathing and concentration during the moving yoga, you also learn to apply this in daily life. You probably notice so much change that you automatically become interested in the other yoga sutras. And if not, then yoga is also still a very nice way of exercising.