Yoga. The word yoga is derived from the verbal root (yuj, meaning “to yoke, harness.”) It has a wide range of applications in the Sanskrit language, from “union” to “team,” to “sum,” to “equipment,” to “conjunction,” and so forth. Early on, it came to also be applied to “spiritual endeavor,” specifically the control of the mind (manas) and senses (indriya). This usage is first found in the Taittiriya-Upanishad, which dates back to the sixth or seventh century B.C.
By the time of the composition of the Bhagavad-Gita – which can be assigned to the third of fourth century B.C. – the word yoga was widely used to denote the Hindu tradition of spiritual discipline, comprising different approaches to Self-realization, or enlightenment. In that formative period, Yoga was still closely allied with Samkhya. This fact is reflected in the Mahabharata epic, which frequently employs the compound samkhya-yoga. In subsequent times, Yago and Samkhya developed into separate philosophical schools, know as Classical Yoga (yoga-darshana) and Classical Samkhya respectively. The position of the former school was codified in the second century A.D. by Patanjali in his Yoga-Sutra, while the latter’s metaphysics was outlined one or two centuries later in the Samkhya-Karika of Ishvara Krishna.
If the schools of Pre-Classical Yoga, as recorded in the Bhagavad-Gita, the Moksha-Dharma, and other didactic portions of the Mahabharata epic, espoused a panentheistic philosophy, Patanjali introduced, as far as one can tell, a dualistic metaphysics. He appears to have rejected the idea that the world is an aspect of the Divine, and made a radical distinction between Nature (prakriti) and the transcendental Self (purusha). Hence Bhoja, in his commentary (Raja--Martanda), felt justified in characterizing yoga as viyoga (“separation”).
However, even thought Patanjali’s system came to be regarded as one of the six classical schools of Hinduism, its dualism prevented it from assuming greater cultural significance. The dominant philosophical orientation within the fold of Hinduism has always been nondualist (advaita). Thus, the schools of Post-Classical Yoga, as recorded in the Yoga-Upanishads and the works of Tantrism and hatha-yoga, reaffirmed the panentheism of earlier times. This is also the essential position of the Integral Yoga (purna-yoga) of Shri Aurobindo.

Different kinds of yoga: ashta-anga-yoga, asparsha-yoga, bhakti-yoga, hatha-yoga, jnana-yoga, karma-yoga, kriya-yoga, kundalini-yoga, lambika-yoga, mantra-yoga, nada-yoga, panca-dasha-anga-yoga, raja-yoga, sapta-anga-yoga, shad-anga-yoga.