The hardness of roller skate wheels is defined by the durometer, measured using Type A, and thus represented by the “A” rating after a number. In essence, the higher the A rating, the harder the wheel will be. Softer wheels (75A – 85A) are generally used for outdoor skating as they will handle bumps, grooves and obstacles better whilst offering better grip on most surfaces.
Comparatively harder wheels (85A – 100A) are more commonly used for indoor skating as they handle smoother surfaces better, allowing for higher speeds, at the cost though of less grip. Softer indoor wheels will offer better grip than hard indoor wheels, depending on how you skate you may find softer wheels better for learning and gradually moving to harder wheels as you progress.
For more information, check out the Wikipedia article which goes into all the different types of durometer ratings.