The word “yoga” comes from the Sanskrit root yuj, which means “to join” or “to yoke”.
Yoga is a practical aid, not a religion. Yoga is an ancient art based on a harmonizing system of development for the body, mind, and spirit. The continued practice of yoga will lead you to a sense of peace and well-being, and also a feeling of being at one with their environment. This is a simple definition.
The practice of yoga makes the body strong and flexible, it also improves the functioning of the respiratory, circulatory, digestive, and hormonal systems. Yoga brings about emotional stability and clarity of mind.
In the practice of Yoga the ultimate aim is one of self-development and self-realization.
Source :http://yoga.org.nz/what-is-yoga/yoga_definition.htm