Yoga is more than just physical exercise. Yoga helps us to recognize and unite what we are: our body, our thoughts, our ego, our emotions, our experiences, our breath... Yoga is a thousands years old Indian tradition that explores anatomy, physiology, psychology and the flow of energy of human beings. It covers all these aspects. Eventually, Yoga shall lead to spiritual enlightenment. The "tools" Yoga uses are diverse: physical exercise, control of breath, philosophical studies etc.
Physical postures are by all means a good approach to living the holistic philosophy of Yoga. For me, at the beginning, Yoga was merely a sequence of postures that would hopefully help me to get rid of my back pain. Today, I am convinced that Yoga is the key to a better health and inner peace.