Anything outside the case that connects to your computer is considered a peripheral.
The keyboard, mouse and monitor are pretty much the bare minimum
you can go with and still be able to interact with your computer. Your choice
in peripherals depends on personal preference and what you intend to do with
your computer.

There are, broadly, two types of mouse: optical and mechanical. Mechanical
mice use a rubber coated ball bearing that contacts the mousepad or other
surface and actually rolls around. Optical mice use a bright light and a sensor
to track the movement of the mouse.
When choosing a mouse, there is generally no reason not to choose an optical
mouse. They are considerably lighter (and as such, reduce RSI) as they have no
moving parts, they are much better at smoothly tracking movement, and they
dont require constant cleaning like ball mice (though it may be wise to brush
off the lens with a q-tip or other soft tool on occasion). Make sure that you
spend money on a decent-quality mouse made by companies such as Microsoft
or Logitech, as lower-end optical mice will skip if moved too fast. Mice of
medium-to-high quality will track your movement almost flawlessly.
Although three buttons are generally enough for operating a computer in
normal circumstances, extra buttons can come in handy, as you can add set
actions to each button, and they can come in handy for playing various video
games. One thing to note is that with some mice those extra buttons are not
actually seen by the computer itself as extra buttons and will not work properly
in games. These buttons use software provided by the manufacturer to function.
However, it is sometimes possible to configure the software to map the button
to act like a certain keyboard key so that it will be possible to use it in games
in this manner.
Wireless keyboards and mice do not now display the sort of noticeable delay
that they once did, and now also have considerably improved battery life.
However, gamers may still want to avoid wireless input devices because the very
slight delay may impact gaming activities, though some of the higher end models
have less trouble with this. The extra weight of the batteries can also be an
inconvenience.