Yoga derives from the sanskrit word yuj - which translated means - yoke/union. 

Yoga is the practice of bringing stillness to the mind, by connecting with the breath we can clear our minds,  Yoga teaches us to develop cessation of the mind through breath control - pranayama. Being able to have this mental clarity and stillness helps attain a higher state of awareness and consciousness. 

In the Western world the depth of yoga can sometimes be missed, yoga is so much more than the physical asana practice of stretching. We can think of yoga as the trees in the image on this page; we see the tall trees but our eyes cannot see the huge network of roots grounding and connecting the trees to the earth, we can think of yoga in the same way, there is so much more than meets the eye. Yoga is primarily a spiritual practice.

Yoga originates from ancient India, it's a 5000-year-old Indian body of knowledge. Yoga is discussed in the ancient foundational Sutra of Hindu philosophy and is perhaps most elaborately mentioned in Patanjali Yogasutra.