What Is it?
a Hindu spiritual and ascetic discipline, a part of which, including breath control, simple meditation, and the adoption of specific bodily postures, is widely practiced for health and relaxation.
Yoga is much more than the poses you see posted all over the internet. Derived from the Sanskrit word "yuj", which means to unite or integrate, yoga is a 5000-year old body of knowledge, all about harmonizing the body, mind and breath through the means of various breath techniques (pranayama), postures (asanas) and meditation.
That said, ask ten different yogis what yoga is for them, and you will get ten different answers. That doesn't mean they are doing different things, it just shows that the practice of yoga, though often practiced in larger groups, is a very personal experience.