Source : Free On-Line Dictionary of Computing

hard link
     
         One of several directory entries which refer to
        the same {Unix} {file}.  A hard link is created with the "ln"
        (link) command:
     
        	ln  
     
        where  and  are {pathnames} within the
        same {file system}.  Hard links to the same file are
        indistinguishable from each other except that they have
        different pathnames.  They all refer to the same {inode} and
        the inode contains all the information about a file.
     
        The standard ln command does not usually allow you to create a
        hard link to a directory, chiefly because the standard {rm}
        and {rmdir} commands do not allow you to delete such a link.
        Some systems provide link and {unlink} commands which give
        direct access to the {system calls} of the same name, for
        which no such restrictions apply.
     
        Normally all hard links to a file must be in the same {file
        system} because a directory entry just relates a pathname to
        an inode within the same file system.  The only exception is a
        {mount point}.
     
        The restrictions on hard links to directories and between
        file systems are very common but are not mandated by {POSIX}.
        {Symbolic links} are often used instead of hard links because
        they do not suffer from these restrictions.
     
        The space associated with a file is not freed until all the
        hard links to the file are deleted.  This explains why the
        system call to delete a file is called "unlink".
     
        (1997-10-22)