a freewheel is a back sprocket that threads on to the hub. on a flip flop hub has two sides threaded; one for 13 and 14t freewheels, the other for larger freewheels. a cassette is a hub with no external threads that uses a driver. you can put a cog on the driver, or you can get a 1 pc driver if you want gearing below 33-12. A freecoaster is a hub that allows you to ride fakie without the crank rotating. when you pedal forward, the driver doesn't engage until you rotate your crank about 1/8 to 1/16 of a circle.