This command do exactly the same as rm : it removes files. If you have read the section about links you know that one file contents can be read and write throw one or more files thant's to hard links. So the unlink command is the exact oposite of the ln command : it removes one link, and if to more links are available for the file contents, it can not be acceeded and will be overwritten by other information.
The number of hard links for a file can displayed by the ls command.
Note that the C programming language primitive for deleting files is :