Yoga is one of the six aspects of traditional Indian philosophy. It has been passed down mostly from teacher to student and there are very few writings about yoga until about 1,000 BCE. The earliest mention of “yoga” is in a vedic script from 5,000 BCE. In that script there is reference to a “long-haired sage” (Rg veda 10).
The most widely known traditional text is Patañjali’s Yoga Sutras which probably date to the 4th century CE. The Yoga Sutras are a collection of 195 short aphorisms about yoga. My favorite modern version of the Sutras is by Alistair Shearer
In modern times, yoga has become three main subjects. Posture (Āsana), Breathing (Prāṇāyāma) and Meditation (Pratyāhāra) The physical postures are the most popular and well known.