An interface is what the user uses on an operating system to make the computer do tasks, like writing a document, or loading a web page. There are two types of interfaces, a command line interface (CLI for short), or a graphical user interface (GUI for short).
Command line interfaces ask the user to type in commands in a text-based environment, not allowing images, like the screen image to the right.
For example, if the user wanted the computer to say "Hello World", he or she would:
have to type in what tells the computer that this is a command (we will use "say")
then have to tell the computer what to "say" ("Hello World" in quotation marks)
then press Enter or Return, giving the command to the computer, which is say "Hello World", so the computer prints Hello World.
This is hard to use for most people, mostly because when the user has to do a long command, he or she can miss a letter or two, so the computer shows an error, and he or she has to do it all over again, which can waste hours.

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