The word yoga derives from Sanskrit and its root ‘yui’ means to unite, to control. Yoga means union - union between body, spirit and mind. This is the unity between your small Self, the Self which you normally identify yourself with, and the Divine Self. Only when you are able to realise they’re one and the same - you will have reached enlightenment. There are many different yoga practices and techniques, like yoga asanas (postures), breathing control, withdrawal from the senses, etc. They are to help your concetration, to prepare your body for mediation. The purpose is always the same - enlightenment.