This page has a great description of what a body scan is. Here is a short excerpt:

“The body scan has proven to be an extremely powerful and healing form of meditation. It forms the core of the lying down practices that people train in Mindfulness-Based Stress Reduction. , bringing an affectionate, openhearted, interested attention to its various regions… Without moving a muscle, we can put our mind anywhere in the body we choose and feel and be aware of whatever sensations are present in that moment.”