What is a freewheel or freehub?

Answer 1

Freehub. When gearing inevitably wears out, with the freehub you only change out the gears (cogs or cogset). With the freewheel you replace the entire assembly, including the ratcheting mechanism and bearings. OTOH freewheel assemblies can be found rather cheap, so the economy of of it isn't that clear-cut. A more definite reason is that freewheels leave more of the axle unsupported, (the bearing is located closer to the centre of the hub) which makes freewheel wheels more prone to broken and bent axles if you want to run 7-8 speeds.