Road bike and mountain bike cassettes for different disciplines, terrains and abilities, also available freewheels from "Shimano", "Cycle Tech" etc. Mountain bike cassettes are adapted to wide variety of altitude levels encountered on an off-road trail. Usually mountain bike disciplines require a major jump in gears and that's why the sprocket sizes on mountain bike cassettes require bigger gaps. Cassettes for road bikes provide smaller jumps between gears and are more suitable for flatter terrain.
You have to keep in mind that cassette and freewheel is not the same although these terms often are used interchangeably. A freewheel allows the rear wheel to turn faster than the pedals and transfer power only from the pedals to the wheel, not in the other direction. To determine if a sprocket is a freewheel or cassette system, remove the rear wheel from the bicycle, find the tool fitting on the sprocket set and spin the sprockets backwards. If the fittings spin with the cogs, it is a cassette system with a freehub. If the tool fittings do not spin with the cogs, it is a threaded freewheel system.