Just FYI for those not command line savvy:
`sudo` gives whatever command comes after it "superuser" privileges. `rm` is short for "remove", as in "delete".
"-r" means to do this deletion recursively. So the deletion happens depth first. Everything in the specified path will be removed. "-f" means to do this deletion forcibly. You will not be asked if you REALLY want to delete things.
"/" is the root directory.
Thus, "sudo rm -rf /" forcibly deletes every single thing on your hard drive, including system files owned by the root user.
So, in short:
DON'T DO IT!
I think this is why tutorials that use the command line sometimes come with "Be Careful!" warnings. You should never run any command without first knowing what will happen when you do so. You can always use the "man" command to find out what a particular command does. As in "man rm" which will give you the manual for the "rm" command. Also, the "--help" or "-h" command switches will sometimes give you more information about how to use a particular command.
–Alex