From YogaAlliance.org

"Answering the question , "What is Yoga?" is challenging and is the subject of extensive academics and philosophical study. As the largest nonprofit supporting yoga, we offer this brief introduction to Yoga, along with perspectives from leading scholars and yogis to help answer this question.

Yoga was developed up to 5000 years again India as a comprehensive system for wellbeing on all levels: Physical, mental, emotional and spiritual. While Yoga is often equated with Hatha Yoga the well-known system of postures and breathing techniques, Hatha Yoga is only a part of the overall discipline of Yoga. Today, many millions of people use various aspects of Yoga to help raise the quality of life in such diverse areas of fitness, stress relief, wellness, vitality, mental clarity, healing , peace of mind and spiritual growth. Yoga is a system, not a beliefs, but of techniques and guidance for enriched living. Among Yoga's many source texts, the two best known are the Yoga Sutras and the Bhagavad Gita. Both explain the nature of -and obstacles to-higher awareness and fulfillment, as well as a variety of methods for attaining those goals.  As in any field, some aspects of Yoga are too subtle to be learned from books or lectures; they must be acquired through experience. Hence Yoga's time-honored emphasis on the student-teacher relationship, in which the teacher helps the student develop a practice that brings deeper understanding through personal experience. .Since the individual experience of Yoga is quite personal and may differ for each practitioner, there are a wide variety of approaches to its practice. Yoga has in recent times branched out in many new directions, some of which are quite different from its traditional emphases. All approaches to Yoga , however, are intended to promote some aspect(s) of wellbeing. As a result, today's practitioners ,have more options than ever as they seek to gain the most from the vibrant, ever expanding field of Yoga."