Yoga literally means to link with the Supreme. Yoga is often thought of as a form of physical exercise, but its bodily sitting postures, asanas, are just the first part of an ancient system of meditation.
The yoga system was originally meant for realizing God, and all varieties of yoga practice are supposed to bring one to that point. Success in yoga ultimately depends on quality of consciousness and the good wishes of the Supreme Person, rather than on our own gymnastic endeavors. The yogic asanas, breath control, and withdrawal of the senses are meant to help pacify the mind so that one can focus all attention on the Supreme within.
Krishna gives His view of yoga practice in the Bhagavad-gita. Since the ultimate goal of yoga is to revive our relationship with the Supreme, anyone in constant contact with God in a mood of loving service (bhakti) is the perfect yogi.