chain whip

First things first: a freewheel is not a cassette. There are several differences between the two. In a cassette, the ratcheting mechanism is part of the hub and is integrated, not screwed on. A freewheel is threaded onto the hub. A way to tell is to look for a flat lockring on top of the smallest cog. If it has splines or grooves, it is a cassette and not a freewheel.

Cassettes are used more typically nowadays, whereas freewheels can be found on older bikes.

You also have to remember that most companies make their freehubs non-compatible with cassettes from other companies. So if you have a Shimano cassette and need to replace a cog, a Campagnolo will never work because they are splined differently.

A chain whip is a tool that will often be included in a decent bike tool kit. As the name implies it has a long handle with a length of chain at the end. It is used as a way to grip the cogs of a cassette allowing you to gain enough tourque to loosen a lockring. Some older cassettes require the use of two chain whips applied in apposing directions to loosen the smallest cog of the cassette which itself acts as a lockring. Unfortunately there are no real alternatives to a chain whip since using anything else will likely screw up the cogs on your cassette.