Durometer is a measurement of an item's hardness.  While several different durometer scales exist, the caster industry most commonly references the Shore A and Shore D scales.  Each of these scales associates a number with a particular hardness.  The higher the number, the harder an item is on that durometer scale.

Knowing a wheel's durometer allows helps us determine the perfect solution for any caster application.  A couple general rules of thumb:

Harder Wheels (Higher on the Durometer Scale)

Are often easier to push or pull

Can usually support more weight

Softer Wheels (Lower on the Durometer Scale)

Are often quieter

Generally provide better floor protection