ln file.one FILE.ONE
This command will create FILE.ONE as a second name for the file file.one. The
two names have equal weight when referring to the file. If one of the names is
deleted with the rm command, the file will remain until all names
referring to the file are removed. Links such as these (hard links)
cannot cross file systems and cannot refer to
directories.
Use of the -s option creates soft links, also known as symbolic links which can cross file system boundaries and can refer to directories. However, if the original file is deleted, a soft link may still exist which then will point to a non-existent file.