Shut down may refer to any of the following:
1. Shut down is a term used to describe the process of closing all software programs in preparation to turn off a computer's power. The operating system is the last program to be close, and it is highly recommended that all computers be properly shut down before being turned off to help prevent data corruption. The picture shows examples of different Microsoft Windows shut down screens. In the to picture, Windows 7 displays a Shutting down message while it is shutting down. Early computers were not capable of turning themselves off, so Windows 98 displayed a message when it was safe to turn off your computer after doing a Shut down.
Is it Shut down, shut down, or shutdown?
All forms of this word are correct. When describing an action or using the word as a verb use "shut down" in your writing. If you're describing the option in the Windows Start Menu use "Shut down." When using the word as an adjective, noun, or a command use "shutdown."
For example, the sentence "Instead of using the Start Menu Shut down option you could use the shutdown command to shut down the computer." uses all forms of this word.