Summary
ref The .sock extension is used by Unix domain sockets as a convention, but not as a requirement.
Unix domain sockets are special files used by different processes to communicate with each other - much like TCP/IP sockets.*
This is explained in the Unix man pages socket(2) and unix(4) - that is, type in the terminal: man 2 socket or man 4 unix.
They are on the web here: socket(2) Mac OS X Developer Tools Manual Page unix(4) Mac OS X Manual Page