To add on, I agree Yoga is tremendous streching and keeping loose muscles. I would also add that its effective for working/training your core muscles(especially). I took a yoga class once for a class project (don't ask), and I was really surprised how tough it was to finish! Some of the poses had me locked in a pushup pose (which gets exceedingly tiring very quickly) to stretching one leg out while focusing on breathing, and everything in between. The last time I went, I sweat. A LOT. Pouring down my face, damp shirt sweating.
Now, to be fair I was really out of shape when I attempted it, and the room was set at a warm tempurature. The point is, I woke up the next day aching in muscle groups I didn't know existed. To say that this was merely a stretching exercise really underwhelms the yoga experience. I certainly used a lot of energy just getting my body tofinish the session.