A jack is a socket for an audio cable to plug into. The corresponding part on the end of the cable is called a plug. In England the tern “jack socket” is often used and some use “jack” to refer to the plug, though this usage is historically incorrect.
Jacks and plugs vary by size, number of terminals, and construction.
Common jack sizes are 1/4 inch (or 6.35 mm) and 3.5 mm.
There can be two, three, or four terminals to connect to wires and ground shielding in the cable. There can also be switch terminals that provide for a normalized connection when no cable is plugged in.
Jacks can be of open or closed construction, and can be panel mount or horizontal or vertical board mount.
The 1/4" jacks commonly used in Kosmo modules are stereo, with five pins:
even though in almost all cases they are used as mono jacks, with the ring terminals unused, and often the tip normal pin will also be unused. In those cases 3- or even 2-pin versions of this style jack can be used. However the 5-pin versions are readily available (more so than 2- or 3-pin) and usually are not much more expensive if at all, so one can buy them in bulk and use them everywhere.