What is a freewheel gear design on rear bicycle wheel?

Answer 1

Bikes with external gears are available in two different rear wheel designs. One is called a freewheel, in which the sprocket cluster together with the one-way clutch that makes coasting possblke is treated as one unit. The other is the freehub design, where the one-way clutch and the sprocket cluster are treated as separate units. With a freewheel you replace the chutch bit too when the sprockets gets worn, with a freehub you only replace the sprockets.

Freehubs also have the bearings further apart which makes them less prone to bending/breaking the axle.