No matter the intricacies of a computer program, its base structure can be summarized as being composed of three key steps: Input, Process and Output, shortened to IPO.

The I in IPO stands for Input. Input is the information that feeds the process, so that it can produce a result according to it. Input can be generated directly by the user, but it can also be the output of another process. When generated by the user, a device is needed in order to provide the user with an interface to enter the information with.

Some common input devices are keyboards, graphics tablets, joysticks, and barcode scanners.

Like I, the O in IPO stands for Output. Output is the final result of the process, and is used either as the input for another process, or represented to the user as the outcome of a program. Some devices used to display the output of a program are headphones, screens, printers, and speakers.

There are also some devices which offer a hybrid functionality, acting both as input and output devices. Some of these are cameras, touchscreens, CD/DVD readers with write functionality, and Internet modems.