So what is it?
A Foam Roller is a foam cylinder of different sizes and densities. Sizes vary anywhere from 12 to 36 inches and are meant for different muscles you are targeting. They are inexpensive and provide an effective solution to stretching.
According to Wikipedia this foam rolling is good for a number of muscles including: gastrocnemius, latissimus dorsi, piriformis, adductors, quadriceps, hamstrings, hip flexors, thoracic spine (trapezius andrhomboids), and TFL.