In a cassette system, the ratchet mechanism that the sprockets fit onto is a part of the hub. So a cassette is the sprockets without the freewheel mechanism. This page lists the tools you'll need to fit and remove your cassette.
If you're not sure whether you have a cassette or a freewheel, spin the sprockets back. If the removal splines in the middle turn with the sprockets, you have a cassette. If the splines stay still while the sprockets turn you have a freewheel.