Yoga is defined in a number of ways. Some of the interpreted definitions commonly used are:
The Union of the individual self with the greater Self.
Search for the Self, quieting the mind.
The handsome union of our will with the will of God (or the Universe).
Yoga traces its roots back to the Vedas ("the originators" which were created and recited and passed down through families for generations in India. There were 4 Vedas:
Rig Veda = hymns
Yajur Veda = describes the rituals
Sama Veda = describes the chants
Atharva Veda = medicine
7 Schools of Indian Philosophy were then created through time, based on the Vedas:
2. Nyaya - School of logic and debate
3. Vaisheshika - Describing the world in small particles, primarily a scientific study into what the world is made of.
4. Sanka (learn more here)
5. Yoga - see below for the different paths of yoga and explanation of each.
6. Mimamsa - has the same beliefs as Vedanta (below) but kept the rituals from the Vedas.
7. Vedanta - considered to be the culmination of the Vedas or "the Vedas end". A non-dualistic view where there is only one reality and the reality is only Brahmin. The world is an illusion. In this, they eliminated all the rituals but gave rise to meditation and individual liberation.
There was also the evolution of Tantra during the 10th Century. However, as Tantra was opposed by the Brahmins, while rooted in the Vedas, Tantra is not considered orthodox.
The Upanishads were derived from the Vedas as well. These were narrative stories of the deities which helped people relate to the various deities. They are the most recent part of the Vedas, that deal with meditation, philosophy and ontological knowledge. The Upanishads played an important role in the development of spiritual ideas in ancient India, marking a transition from Vedic ritualism to new ideas and institution. Their central ideas are the spiritual core of Hinduism.
(Raja = Royal Yoga) The 8 fold path as taught by Patanjali in around 200 BC, based on 196 Yoga Sutras. Man's true nature is divine. We are unaware of this divinity because we falsely identify ourselves with our body, mind, and objects of this world.
"Most religions teach a person what to do. Raja Yoga teaches one how to be." ~ Swami Rama
Hatha Yoga is the use of the physical posture, breath control, and self-purification. Hatha Yoga is not only a philosophy but a science. Working with the body, we learn to control the mind.
the human body or mind is a field of energy. Energy flows through the nadis (energy channels).
Note: As Kundalini Yoga deals with the movement of energy within the body, it is very important to have an experienced yoga teacher to guide you.
Used for concentration during meditation, this helps guide the aspirant inward, keeping attention focused and adding spiritual energy to the process of meditation.
Every path acknowledges the selfish ego is a hindrance in life. Through selfless service (karma yoga), devotion (bhakti yoga), and a clear understanding of the teachings (jnana yoga), an aspirant can reduce the bondage created by the ego.
(To learn more, see also The Bhagavad Gita)