From Wikipedia: Yoga is a group of ancient spiritual practices originating in India. As a general term in Hinduism it has been defined as referring to "technologies or disciplines of asceticism and meditation which are thought to lead to spiritual experience and profound understanding or insight into the nature of existence."
Outside India, Yoga is mostly associated with the practice of asanas (postures) of Hatha Yoga or as a form of exercise, although it has influenced the entire Indian religions family and other spiritual practices throughout the world.
Hindu texts discussing different aspects of yoga include the Upanishads, the Bhagavad Gita, the Yoga Sutras of Patanjali, the Hatha Yoga Pradipika, the Shiva Samhita, and many others.
Major branches of Yoga include: Hatha Yoga, Karma Yoga, Jnana Yoga, Bhakti Yoga, and Raja Yoga. Raja Yoga, known simply as Yoga in the context of Hindu philosophy, is one of the six orthodox (āstika) schools of thought, established by the Yoga Sutras of Patanjali.
Swami Vivekananda translates the sutra as "Yoga is restraining the mind-stuff (Citta) from taking various forms (Vrittis)." Gavin Flood translates the sutra as "yoga is the cessation of mental fluctuations".
Patanjali's writing also became the basis for a system referred to it as "Ashtanga Yoga" ("Eight-Limbed Yoga"). This eight-limbed concept derived from the 29th Sutra of the 2nd book became a feature of Raja yoga, and is a core characteristic of practically every Raja yoga variation taught today. The Eight Limbs of yoga practice are:
(1) Yama (The five "abstentions"): nonviolence, truth, non-covetousness, chastity, and abstain from attachment to possessions.
(2) Niyama (The five "observances"): purity, contentment, austerities, study, and surrender to god.
(3) Asana: Literally means "seat", and in Patanjali's Sutras refers to seated positions used for meditation. Later, with the rise of Hatha yoga, asana came to refer to all the "postures"
(4) Pranayama ("Lengthening Prāna"): Prāna, life force, or vital energy, particularly, the breath, "āyāma", to lengthen or extend
(5) Pratyahara ("Abstraction"): Withdrawal of the sense organs from external objects.
(6) Dharana ("Concentration"): Fixing the attention on a single object
(7) Dhyana ("Meditation"): Intense contemplation of the nature of the object of meditation
(8) Samadhi ("Liberation"): merging consciousness with the object of meditation