The ratchet can be used to move a toothed wheel one tooth at a time. The part used to move the ratchet is known as the pawl.
The ratchet can be used as a way of gearing down motion. By its nature motion created by a ratchet is intermittent. By using two pawls simultaniously this intermittent effect can be almost, but not quite, removed.
Ratchets are also used to ensure that motion only occurs in only one direction, useful for winding gear which must not be allowed to drop. Ratchets are also used in the freewheel mechanism of a bicycle.