What are ratchets?

Some articles on ratchet, ratchets:

Socket Wrench - Accessories
... called "extender arms", attach to a socket on one end and a ratchet on the other end of the extension ... by turning extension + socket with or without ratchet ... They are used with extensions and ratchets for turning a bolt or nut at a difficult to access location ...
Socket Wrench Types - Pass Through Sockets and Ratchets
... Each pass-Through socket, within a given socket set, is designed to be used with a "special" ratchet that fits on the outside of the socket and not to the middle of the socket ... Pass-Through sockets and ratchets are built for 1/4", 3/8" and 1/2" sets in both SAE and metric sizes ... By using a fine tooth ratchet and socket system that allow a pass-Through ratchet to be used as a conventional ratchet handle with interchangeable 1/4-inch and ...