The word "Yoga" came from the Sanskrit word "yuj" which means "to unite or integrate." Yoga then is about the union of a person's own consciousness and the universal consciousness.Yoga was likened to as a tree with a living trunk, roots, branches, flowers and fruit in ancient times. In addition to its most famous branch - Hatha yoga - yoga consists of six major branches including Bhakti, Jnana, Karma, Raja and Tantra forms of yoga.
The Aim of Yoga
According to the Yoga Sutras of Patanjali, the ultimate aim of Yoga is to reach "Kaivalya" (emancipation or ultimate freedom). This is the experience of one's innermost being or "soul" (the Purusa). Then one becomes free of chains of cause and effect (Karma) which tie us to continual reincarnation. In Kaivalya one is said to exist in peace and tranquillity, having attained absolute knowledge of the difference between the spiritual which is timeless, unchanging and free of sorrows, and the material which is not.
This is considered desirable as life is analysed as ultimately full of sorrows and pain- even pleasure and joy leave pain and loss when they have gone as nothing in the material world is permanent.
Yoga is therefore a spiritual quest. However, along the path of yoga, the aspirant also gains health, happiness, tranquillity and knowledge which are indicators of progress and an encouragement to continue their practice. Buddhism and other Eastern spiritual traditions use many techniques derived from Yoga.
The Philosophy of Yoga
The philosophy of Yoga comes from many sources and has been presented in many fashions with differing emphasis depending on the understanding of the author.
The Vedas and Upanishads give some of the earliest references to the paths of yoga. These scriptures form the basis of Indian religious practices but contain many varied references to yoga and other things.
There are the Puranas, also ancient, which deal with the nature of the universe.
Famous epics such as the Ramayana and Mahabarata contain stories of the gods and lectures on moral and philosophical subjects with references to yogis and yogic practices.
The Bhaghavad Gita is a particularly famous part of the Mahabarata which contains a detailed discourse on yoga by Krisna to Arjuna.
Other texts such as the Hatha Yoga Pradipika are more "technical manuals" of yoga which go into detail on technique as opposed to just the theory.
In general all these texts discuss Yoga from the particular standpoint of the authors and the paths to Yoga they have followed. In many ways this subject can be confusing for lack of a clear overview. This need is answered in the Yoga Sutras of Patanjali.
The Paths of Yoga
There are said to be 4 main paths (Margas), according to the Bhagavad Gita, by which to reach the ultimate goal of Yoga - "Kaivalya." There is the path of Knowledge (Jnana Marga) in which one learns to discriminate between what is real and what is illusory, the path of selfless work (Karma marga), the path of devotion (Bhakti Marga) and the path of control of the mind (Yoga Marga) where all the activities of the mind and consciousness are studied and brought under control. From these have come the various paths of yoga which can be followed.
Raja yoga involves mastery of the mind and senses in Samadhi; essentially the advanced aspects of Patanjali's astanga yoga.
Hatha yoga is the yoga of the will which involves cultivating ones energy to arouse Kundalini primarily by means of asana and pranayama.
Mantra yoga involves reciting sacred syllables to reach perfection.
Laya yoga involves absorption in god to experience ultimate bliss.
Bhakti yoga requires absolute devotion to god to achieve the ultimate goal.
Karma yoga achieves this through selfless work without thought of personal reward.
Jnana yoga is the yoga of knowledge cultivating the discrimination between spiritual reality and the illusion of the material world.
It must be realised that there are no clear cut boundaries between these various paths and all draw on the practices and philosophy of the others; effectively all paths have the same goal and "tread the same terrain." They are different views of the same topic.