It's only on the most superficial level that yoga is 'about' the body in an exercise context, or even about stress reduction, though those are popular and wholly appropriate places to begin.  Nor can we say, exactly, that yoga is a kind of spirituality or religion.  It isn't concerned with gods or theologies.  It's only concern is humanity.

There is nothing so deeply revolutionary, so kind and yet so moving, as being concerned with our own humanity.  As exploring the cave of our own heart.

This is a process of becoming more and more our selves.  More and more tacitly alive.