The "command line" is an ancient way of giving a computer a series of small, self-contained orders (think of those science fiction movies where teenage geniuses type long strings of green text onto black screens). To use a command line tool, the user types a command into a window called a terminal emulator, hits the return or enter key, and then receives a textual response in the same window. Windows, Linux and Apple desktop computers still let you run software using this interface, and even some mobile phones can do the same with the right app. The command line can be used to run software pre-packaged with your operating system. Some downloadable programs, especially technical utilities, use the command line instead of a more familiar "icons and buttons" user interface. The command line needn't be scary, but it does require you to type in exactly the right set of letters and numbers to get the correct result, and it's often unclear what to do if the responses don't match your expectations.