Many people come to yoga looking for an alternative type of exercise for various reasons; to lose weight, gain flexibility, build strenth, to get their hearts pumping, or to deal with stress, anxiety and other types of mental/emotional issues. All legitimate reasons, but what makes (hatha) yoga different from a stretching class or a gymnastics session?
Different schools of yoga have slightly different takes on this. For example, Ashtanga, an athletic flowing series of positions lend great significance to Bandhas (physical locks of certain muscles and muscle groups) and breath. The Ujjayi breath (a specific type of slow, controlled breathing) is the main focus of the practice. Movements are controlled and performed in time with the breath. The focus on the breath and the bandhas encourages the yogi to integrate the physical movements with a calm and controlled mind.
The Iyengar school, which is very much concerned with correct alignment in the asanas, might suggest that the difference between yoga and other types of exercise is that yoga asanas have been developed over thousands of years. By aligning the physical body in these asanas one can systematically stimulate every muscle, organ, nerve, gland, system and cell in the body allowing the subtler bodies (energetical & spiritual aspects of ourselves) to be accessed.
Some styles emphasis the spiritual aspects of yoga such as chanting, meditation, readings and affirmations, some focus on the energetics of yoga and dedicate the practice to raising our core prana/energy (Kundalini) which resides at the bottom of our spine. Some will focus on cleansing the body from the inside out by practicing in high temperatures, encouraging sweating to detoxify and burn of the toxins.
All styles recognise these various aspects of yoga, but identify themselves by their emphasis of certain aspects of the practice over others.
What makes yoga different to other types of exercise is how the unity of the mind, body and spirit is the focus, through it’s rainbow of styles. Yoga restores and maintains balance within all the different bodily systems, bringing health and peace to the mind as well as the body, rather than just the body, or just the mind. That’s not to say that one needs to be interested in this union to attend a class; yoga does have great results on the physical and on relaxing your body and mind, but what makes it unique is that it sets the yogi up on a path, if they wish to journey on it, to achieve at least a snippet of inner peace and acceptance.