Freewheel vs Cassette
A freewheel is a block of gear cogs that screw onto your hub motor or wheel. On a Cassette the gear cogs slot in one by one onto the cassette body and held in by a nut.
Both freewheels and cassettes come in a different numbers of gear cogs, most commonly being 7-speed and single-speed.
Mostly all of our kits come with Freewheels instead of cassettes because it reduces the rear dropout spacing, allowing our motors to fit into a larger range of frames.
Here is a great youtube video explaining things in more detail: