The freewheels on this page are singlespeed freewheels. Nowadays you mainly come across these on BMX bicycles (usually M30 thread), singlespeed bicycles and on cheaper city bikes without gears.

You can recognize the rear hubs of those bikes by the simplicity with a set of threads on the right side of the rear hub. A traditional freewheel as seen here cooks on that same wire.

A singlespeed freewheel is attached to a hub by screwing it onto the external thread of the singlespeed hub. By turning and then while cycling, the singlespeed freewheel is tightened onto the thread of the singlespeed hub, so no tools are needed to install a singlespeed freewheel. Use a good mounting grease when mounting a freewheel.

The outer side of a freewheel can rotate freely in one direction relative to the inner body, but not the other. Most singlespeed freewheels achieve this by means of pawls that engage a ratchet.
Freewheel Threading

The standard ISO thread for freewheels is 1,375 x 24 TPI. But there are several other types of freewheels on the market.
Italian wire freewheel 1.378" x 24 tpi 35 x 1.058 mm
ISO thread freewheel 1.375" x 24 tpi 34.92 x 1.058 mm
British Wire Freewheel 1.370" x 24 tpi 34.80 x 1.058 mm
French wire freewheel 1.366" x 25.4 tpi 34.7 x 1 mm
Metric BMX thread freewheel 1.181" x 25.4 tpi 30 x 1 mm

The older British and Italian standards use the same thread pitch for the singlespeed freewheel but a slightly different thread diameter, and are generally interchangeable. However, for strong cyclists it is best not to mix and match - singlespeed freewheels sometimes strip the threads of aluminum hubs. A French freewheel may start spinning on an ISO/British/Italian hub but will quickly lock up (so won't fit). An ISO/British/Italian freewheel skims over the threads of a French wired singlespeed hub and slides forward if an attempt is made to use it. Never force a freewheel! You will damage the hub.

Freewheels are not very common anymore, but for singlespeed bicycles, bmx and other bicycles, brands such as BLB, Sturmey Archer, Dicta, White Industries and Shimano will always make a freewhee