A symbolic link (or symlink or soft link) is a special type of file which contains a reference to another file or directory. If a symlink is deleted, the target file or directory is still there. A symlink can still exists even if its target is deleted. Then, the soft link is orphaned/broken.
To create a symlink :
fool@localhost:~$ ln -s path_to_the_target_file link_name
A hard link refers to the specific location of physical data.
To create a hard link :
fool@localhost:~$ ln target link_name
The hard link points to the same data than the file considered as the target. If you remove a hard link, you remove the data as well.
In the Debian coreutils package, there are other commands for managing soft links and hard links.
Instead of using ln without options, link can do the job :
fool@localhost:~$ link file_name link_name
To see which target file a symlink points to, you can use readlink :
root@localhost:~# cd /etc/rc2.d && readlink S30gdm
The coreutils package is installed by default at the time you have installed Debian.