Cassettes are distinguished from freewheels in that a cassette has a series of straight splines that form the mechanical connection between the sprockets and the cassette compatible hub, called a freehub, which contains the ratcheting mechanism. A freewheel is a cluster of sprockets built around a ratchet and threads directly on to the hub. Most cassette systems will use a cassette hub as these allow for a wider bearing spacing and increased axle life, you can also fit a larger quantity of sprockets onto a cassette hub with 11 speed systems now becoming available. With cassettes you must use the appropriate spacing for your gear levers and rear mech, Shimano and Campagnolo are not interchangeable so you will need to match the cassette to your levers for smooth and precise shifting. Sram and Shimano chains and cassettes will work together as they use the same spacing.