Foam rolling is a self-myofascial release technique that is used to help release tightness and tension that is built up during running, and weight lifting. According to wikipedia this causes autogenic inhibition to improve soft tissue extensibility of different areas of the body. This causes the muscle to relax and allows activation of the antagonist muscle.
You can use the foam roller on you calfs, upper back, lower back, hips, quadriceps, hamstrings and TFL/iliotibial band.
Use the roller roll it under each muscle group until a tender area is found, and maintaining pressure on the tender area for 30–60 seconds. And repeat no more than 10x for each area.