A freehub is like a coaster brake (pedal back brake) but WITHOUT the brake action.
So you can roll backwards without the pedals moving.
The picture above shows a normal ratchet freewheel.
I have been riding on a freehub for 15 years now. Freecoaster is an old name for a freehub, back when coaster brakes were still in use.
Freehubs are big thick things, as they have the freehub guts in there, like a cassette hub.