Inline wheels are basically the same as quad wheels as far as what they are made of, hardness to surface, etc. The biggest different is that inline wheels are about a third of the width and about twice as tall. The thinner wheel makes the push faster for skaters in the sport and the wheels being in a line helps the wheels go faster. The measuring is that same as other types of wheels too, the lower the durometered number the softer the wheel and the hgiher the number the harder the wheel. Remember though the lower the number the most likely the wheel can be used outdoors.