The word Yoga is derived from the Sanskrit root "yuj", i.e. "to yoke" [to the spirit] meaning "union of the Individual Soul (Atman) with the Universal Soul (Brahman). "Atman" and "Brahman" are Hindu idealogical terms, and are used as a reference for the mind, whereas there truly is only Oneness.

Yoga embraces a wide range of disciplines whose ultimate goal is the joining of body,mind and spirit, a conscious unification into Oneness.

Yoga is one of six classic systems of Hindu philosophy whose roots date back 5,000 years. There are four classic Yogas, and they are progressive in nature; i.e., Karma Yoga (right acts and actions), Bhakti Yoga (devotion), Raja Yoga (meditation) and Jnana Yoga (inner wisdom or enlightenment). The classic four yogas have expanded into several other forms of Yoga. 

Hatha Yoga (worshipful poses), for example, is part of Raja Yoga training. Some of the other forms of Yoga are Nada Yoga (music), Mantra and Japa Yoga (chanting and on beads)and Kundalini Yoga (study of the psychic centers or chakras).

In the Western world, the most popular form is Hatha Yoga, (Sanskrit for "Union of Force"), a form of Yoga that stresses mastery of the physical body as a gateway to attaining spiritual perfection. Hatha Yoga traces its origins to Gorakhnath, the legendary 12th-century founder of the Kanphata Yogis.

-source: www.bikramyoga.com