There are many different methods and schools of yoga.  When most people think of yoga, they're thinking the physical yoga taught in yoga classes.  This type of yoga is called hatha yoga, which is just a part of the whole of yoga teaching.

Yoga described by the sage Patanjali in the Yoga Sutras is a wholistic method for moving the practitioner toward enlightenment.  It involves lifestyle suggestions, the ability to gain stillness of body and mind, concentration, meditation, and deep contemplation.

There are many translations and commentaries on the Yoga Sutras.  My top recommendation is my own guru's book:  The Spiritual Science of Kriya Yoga.  I've also read and referred to How to Know God : The Yoga Aphorisms of Patanjali by Swami Prabhavananda and Christopher Isherwood.

For hatha yoga videos try Amazon's Yoga Corner.