A peripheral is "an ancillary device used to put information into and get information out of the computer".

There are three different types of peripherals: input devices, which interact with or send data from the user to the computer (mice, keyboards, etc.), output devices, which provide output to the user from the computer (monitors, printers, etc.), and input/output devices that perform both functions.
Touchscreens are an example that combines different devices into a single hardware component that can be used both as an input and output device.