Pranksters on a bunch of internet forums will tell uninformed computer users to type this into the Terminal to fix their computer problems. Once these poor bastards hit the return key and type in their password the system starts erasing everything on the hard drive. This command is only for use with Un*x-like operating systems (Mac, Linux, and BSD but not Windows.) The Windows version of this command is 'format c:'
Uninformed User- "Hey I need help with something."
Troll -"Type 'sudo rm -rf /' into the Terminal and hit return, and then enter your password"
Uninformed User- "WHAT THE FUCK!?!?!?"