Cassettes and freewheels are mechanically different and should not be mixed up. A rear wheel hub accepts either a cassette or a freewheel - one or the other. The terms are not interchangeable.
Confusion often arises because these drivetrain components look alike when installed:
Bikemanforu's video guide can help you identify the cogset your bike uses. Follow the tutorial instructions and consider these general guidelines.
Freewheel - External Theading System
Cassette - Freehub System
Some freewheels and cogs on older bikes require other removal techniques - if the tool is no longer made or the part is a transitional type component manufactured during the evolution of bicycle drivetrains in the quarter of the 20th century. For more how-to videos on bicycle repair, check out BikemanforU's award-winning YouTube channel