The word yoga is derived from the root"Yuj" which means integration or union or to join. As far as human being is concerned, the term yoga refers to the integration of a person, which we divide broadly into three important aspects the physical, the mental and the spiritual. A personality is a continuum and one part/aspect can not be separated from another.
Therefore what effects one will automatically affect the other too. The science of yoga begins to work on the innermost aspect of the personality, the mental aspect by yama, niyama, dhyana, and the physical aspects by asanas, pranayam, and the spiritual aspect by pratyahara, dharana and Samadhi.
The history of yoga may go back anywhere from two to eight thousand years ago, depending on the perspective of the historian.The word yoga was first mentioned in the oldest sacred texts, the Rig Veda. The Vedas were a collection of texts contained songs and rituals used by Brahmans, the Vedic priests. Yoga was slowly refined and developed by Vedic priests, who documented their practices and beliefs in the Upanishads, a huge work containing over 200 scriptures. The most renowned of these Yogic scriptures is the Bhagavad-G?t?, composed around 500 B.C.E.
However, its basic text is the "Yoga Sutras" written by Patanjali (c. 2nd century BC?). The Yoga Sutra is considered the fundamental text on the system of yoga, and yet you won't find the description of a single posture or asana in it. This is a guide for living the right life. The first systematic presentation of yoga was Patanjali's Yoga-S?tras. Written some time in the second century, this text describes the path of Raja Yoga, often called "classical yoga". Patanjali organized the practice of yoga into an "eight limbed path" containing the steps and stages towards obtaining Samadhi or enlightenment. Patanjali is often considered the father of yoga and his Yoga-S?tras still strongly influence all styles of modern yoga. The heart of Patanjali's teachings is the eightfold path of yoga. It is also called the eight limbs of Patanjali, because they intertwine like the branches of a tree in the forest. These aren't commandments (although they sometimes sound like them), laws, or hard and fast rules. Astanga Yoga is the most systematic branch of yoga. Astanga Yoga means 8-limbed yoga or 8-runged yoga because there are eight main parts to it. It has been around for thousands of years.
Yoga is very ancient discipline. With time, a discipline can become enveloped in superstitions and misconceptions, especially when knowledge is passed on orally, so that the original meaning can be distorted. The first misconception is the belief that yoga is only for a select few, this may be raised from the fact that some of the ancient yogis were recluse who renounced their worldly lives and went off into forests. The yogic science was slowly evolved in India and developed by ancient sages all over world. And now yoga journey will end in United States of America with its patent.
Some people (including few yogis) have wrong notion that yoga is from of treatment, NO it is not. Yoga may be used as preventive treatment, but certainly not for a complete cure. Essentially, yoga is a lifestyle, you can't practice asanas in yoga class, feel the stretch, and then go home and dream about female secretary, play with your kids, cook a meal, and cheat on your taxes. There is more to yoga than that - yoga can help you cultivate body, mind, and spiritual awareness. In the West, outside of Hindu culture, "yoga" is usually understood to refer to Hatha Yoga. Hatha Yoga is, however, a particular system propagated by Swami Swatamarama, a yogic sage of the 15th century in India.In the West followers of yoga have taken a less spiritual approach and focusing more on the physical part of it that is stretching and breathing.