The practice of Yoga was created around 5000 years ago in India, this exercise is to promote the coming together of mind, body and spirit. This exercise requires a mix of strength, flexibility but mostly body awareness. There are many different styles of yoga, Hatha is the most common, Astanga is a more challenging workout.
The upside to yoga just like pilates is that it gives you a core body workout without causing any real strain. If you aren't flexible then yoga is the thing for you. The downside to Yoga is that its tricky to achieve some of the positions at first.