The word asana is usually translated as "pose" or "posture," but its more literal meaning is "comfortable seat." Through their observations of nature, the yogis discovered a vast repertoire of energetic expressions, strong physical effect on the body but also a concomitant psychological effect. Each movement demands that we hone some aspect of our consciousness and use ourselves in a new way.
While a dancer's or athlete's internal impulses result in movement that takes him into space, in asana practice our internal impulses are contained inside the dynamic form of the posture. When you witness a yoga practitioner skilled in this dynamic internal dance, you have the sense that the body is in continuous subtle motion. What distinguishes an asana from a stretch or calisthenic exercise is that in asana practice we focus our mind's attention completely in the body so that we can move as a unified whole and so we can perceive what the body has to tell us. We don't do something to the body, we become the body. In the West we rarely do this. We watch TV while we stretch; we read a book while we climb the StairMaster; we think about our problems while we take a walk, all the time living a short distance from the body. So asana practice is a reunion between the usually separated body-mind.
A huge diversity of Asanas
The vast diversity of asanas is no accident, for through exploring both familiar and unfamiliar postures we are also expanding our consciousness, so that regardless of the situation or form we find ourselves in, we can remain "comfortably seated" in our center. Intrinsic to this practice is the uncompromising belief that every aspect of the body is pervaded by consciousness. Asana practice is a way to develop this interior awareness.
Apart from the vibrant health, flexibility, and stamina this unified body-mind brings us, living in the body is also an integral aspect of spiritual practice. The most tangible way that we can know what it means to be compassionate or not grasping is directly through the cellular experience of the body. The most direct way that we can learn what it means to let go is through the body. When we have a self-destructive addiction--the impulse to overeat or to take drugs--this happens through the entrenchment of neurological and physiological patterns in our bodies. And on a more basic level, it's hard to feel focused and purposeful when our bodies are full of aches and pains or burdened with illness and disease.