Coupling nuts are elongated internally threaded nuts also known as extension nuts.  These are used to connect two male threaded parts like threaded rod or hanger bolts. 

High Nuts

High nuts have an increased height from standard nuts.  This larger surface area is better for wrenching in applications like automotive and industrial equipment industries. 

Square Nuts

Square nuts have an outer square shape keeping them from rotating or loosening once in a channel or square hole.  These nuts are easier to grab and tighten with a wrench.

Acme nuts have square threads that mate with acme threaded rods, studs, and screws.

Cap nuts, also known as acorn nuts, have a domed head which protects the screw or bolt from stripping.  The acorn nut provides a finished look to the end of the bolt when applied.  Cap nuts can be reused and removed easily. 

Wing nuts are tightened and loosened by hand.  These nuts are typically used in applications where adjustments or disassembly are frequently required.  Wing nuts can be used in panels, marine hardware, and furniture applications to name a few.