Or do you mean the directory on a disk set as a sharepoint?
If you mean this, then it is a directory (or entire disk) which is made accessible for other users via sharing over a network.
Every Mac OS X client has the user's Home folder set as a sharepoint, if you enable Filesharing, and, ofcourse, that user's name and password are known.
Also the Public folder within the Home directory is set as a Sharepoint for guests.