What it isn't?
A lot of people misinterpret Yoga as a bunch of people bending themselves into weird positions. In fact there is a whole Yogic Science and philosophy behind it. When westerners think of Yoga they are actually thinking of a form of yoga called Hatha Yoga.
What it Might be
A highly recommended and really thoroughly written page on what Yoga is or isn't <shakes finger at aristotle> check out Swamij's interpretation.
Wikipedia has also done an extensive job explaining what Yoga is, check that out as well.
The wiki hosts other specific explanations on what and how to integrate Hatha Yoga and its counterpart meditation into your day to day routine.
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