Close up the differences between freewheels and cassettes include how they mount to the wheel. This will help people identify whether they have a cassette or a freewheel.
Freewheels usually have 5 – 7 cogs and screw on to the hub. Modern cassettes (like Shimano Hyperglide) usually have 7 – 11 cogs and are held onto a ratcheting freehub body with a lock ring. There was an older Shimano cassette called Uniglide that was held on the freehub body with the smallest cog acting as the lock ring. Uniglide cassettes are not very common anymore.
Source : RJ The Bike Guy