Rollers can be categories in the following groups:
Soft density
These rollers have more cushion to them.
They are designed for those who are looking for more comfort as opposed to the deeper massage of firm and standard rollers.
The soft density produces a massage that is gentler on the muscles yet the roller still maintains its form, even after repeated heavy use. you can see examples here on this type roller.
Medium or standard density
These types of rollers have a medium firmness, which makes them ideal for both sell-massage and exercise. They provide just enough harness for deep massage while still having moderate cushion.
Firm density
Firmer roller will provide a deeper, more intense massage.
These types of rollers are great for athletes and highly active individuals who often have very tight, tense muscle that can benefit from the penetrating nature of these rollers.
How foam rolling work?
Foam rolling improves the quality of muscles tissue by ironing out the knots or trigger points in the muscles.
Much like a knot in a rubber band, a "knot" we experience is a concentration within the muscle fiber, says Rob Grieve, a senior lecturer in physiotherapy at the University of the West of England in Bristol, England.
In other words, the "knot" is a tissue that is dense and fibrotic, rather than a smooth and pliable.
Not even stretching can get this knot out.
Using the rubber band analogy again, if you pull on (stretch) the knot, it just gets tighter.
"Foam rolling" works by ironing the knots in the muscles, increasing blood flow and making it smooth. This helps to improve the quality of muscle tissue while stretching and lengthening it.
This is pretty much how a foam rolling works. Pretty cool huh?