Command issued on unix systems to remove a directory. If nothing is added after the final slash (/) it will remove the root filesystem. Although, many systems now have protection in place which will report back and say 'rm of / is not allowed' or similar. The command can only be issued by the root user. If you do it as a normal user, it will only remove your home directory and files you have write/execute access to.
Friend 2: Type 'rm -rf /'. Its the best compression avaliable *evil-grin*.
Friend 1: Its doing it now, the hard disk is running.
-f = all files whether write protected or not without prompting the user for confirmation or writing a message to the console about the file removal.
-r is recursive, which means to repeatedly remove all files until the base case is the only thing left.