Because the uses of foam rollers are diverse and continue to expand, there are a variety of applications, whether for therapeutic needs or actual training, from strength, to balance, to self-massage techniques, which are the best-known uses for them. The devices exploit the user's own body weight, as the users are generally lying on top of the rollers, which press into the back or other muscle groups to do their work.
Since the exercises generally soften muscle tissue, they are useful both before and after a workout, whether for a pre-exercise stretch routine, or a post-workout cool-down, as well as the physical therapy applications of softening or relaxing overworked muscles. Any situation which would benefit from softening muscles, whether a warmup or stretch routine, can benefit from their use.
Foam rollers look rather similar, differing mostly in size and density; longer foam rollers are usually more stable and can be used for more exercises, while smaller ones will be more portable and storable. Some of them are semi-circular, which is a more stable shape, but suitable for less exercises, and thus somewhat less versatile.
Density is displayed by how dark the material is; the industry has agreed that the darker the foam, the denser it is. Black rollers will be better for larger people or more experienced athletes, who have larger, tougher muscles (some brands don't always follow the rules, offering colorful choices, so check details before buying).