Yoga literally means "union". The union in Yoga, if considered at the gross physical level, is between the upper brain and the lower brain, the conscious with the subconscious. At a subtle level the union of yoga is between one's individual consciousness and cosmic consciousness.
The happiness of individual consciousness is attached to all kinds of things which appear and disappear : posessions, relationships, positions, pleasures, pains,... Merging with cosmic consciousness is a search for eternal happiness, independent of what happens. The yogi understands that the blissful nature of his or hers essential being can only be fully experienced when dropping all other desires. These desires largely originate in the subconscious mind, which must be brought into the open by first dropping the conscious mind. Then the subconscious mind can be fully purified of its fears, pains and irritations, which opens the path to cosmic consciousness.
Yoga presents a system that creates a state of unification of mental processes and consciousness. Yoga is based on particular disciplines and exercises through which union of individual and cosmic consciousness may be obtained by anyone who chooses to adhere to the prescribed system. Yoga not only describes all mental states, aspects and dimensions possible within the individual, but also advocates practical games to be played with the mind to control its functioning and to achieve peace. Meditation is only one aspect of Yoga.
Essentially the yogi will choose according to his temperament (the doshas) to follow one of the 3 yoga paths. All three in combination or not, may lead the yogi to obtaining the union between individual and cosmic consciousness:
||The path of pure spiritual devotion, love and surrender.
||Also called Gyana, it is the path of right knowledge, reaching the ultimate truth through cessation of mental modifications by negation.
||The path of selfless action.
There are many yoga traditions and practices, which in essence all have the same goal : the union with the divine.
The overall tradition :
Tantra Yoga (Maha Y)
The four main traditions :
Laya Yoga (Kundalini Y.)
Yoga and Indian psychology
All these traditions are based on the unique Indian Psychology. Also read the abstracts on "Mind, Ego and Intellect" and on "The five Koshas" by Wil Geraets.
You can also read : How to meditate ? - What is the best yoga ?