where <old name> and <new name> 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.
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.
Unix manual page: ln(1).