Yoga is an ancient form of physical and mental exercise that is still popular today. It involves lots of stretching and holding poses with funny names, like "downward dog."
The kind of yoga that most westerners practice is based on "hatha yoga," the most familiar part of a complex Hindu system of meditation and spiritual practice. Yoga is an extremely popular form of exercise today, but it's been around for centuries. The word yoga was first used in English around 1820, when western intellectuals were first becoming interested in Hindu philosophy — it's a Sanskrit word that means "union."