Yoga refers to traditional physical and mental disciplines originating in India, to the goal achieved by those disciplines, and to one of the six orthodox (āstika) schools of Hindu philosophy

Major branches of a yoga include Raja Yoga, Karma Yoga, Jnana Yoga, Bhakti Yoga, and Hatha Yoga, Raja Yoga, is known simply as yoga in the context of Hindu philosophy, is part of the Samkhya tradition.Many other Hindu texts discuss aspects of yoga, including the Vedas, Upanishads, the Bhagavad Gita the Hatha Yoga Pradipika, the Shiva Samhita and various Tantras.

The Sanskrit word yoga has many meaning, and is derived from the Sanskrit root yuj, meaning to control, to yoke or to unite.Outside India, the term yoga is typically associated with Hatha Yoga and its asanas (postures) or as a form of exercise. A practitioner of Yoga is called a Yogi (unisex term) or Yogini (for female).

(source:http://en.wikipedia.org/wiki/Yoga)