A cassette is held on to the hub by means of a special nut called a lock ring . The lock ring uses a special socket that costs <$10 (first pic). You also need a chain whip to brace the whole works while you unscrew it (second pic, chain whip on right, wrench with socket on left. Note skewer is on to stabilize the nut for initial turn.) There is some technique involved and you should be careful about keeping the cogs and spacers in proper order. You will not find any spring loaded surprises. Really, it's easy and removal is about the only way to thoroughly clean a cassette. You can do this.
Park tool explains it in great detail
here.
This is a freewheel
This is the removal procedure for a cassette