A bicycle cassette is the part that sits in the middle of the rear hub. The cassette is equipped with x number of chain drives where the chain can be moved between these drives in a simple way through a rear derailleur. These cassettes are usually available with between 6-12 sprockets (gears). Take care of your cassette and don't let it get filled with dirt and oil. Feel free to use a good chain wax for lubricating your bicycle chain instead of a classic oil. This is to prevent the build-up of dirt on your chain and cassette. If you are going to change the cassette, it is important to know what cassette body you have on your rear wheel. Some examples are HG-body, XD-body or Micro Spline body.