In this article I attempt to give a very high level overview of computers, devices and accessories and how they interact with each other. This information is for people who are starting with computers and want to learn the basics. It's important that you understand these high level concepts before moving on to more in-depth tutorials.
The computer can be thought of as the brain. All the other devices that plug into the computer are inputs and outputs – either either input some data, output some data, or both. The computer is independent of these devices and can run by itself. For example, the computer does not need a monitor to run – the monitor is just there for the human operator to see what is going on.
Terminology: Sometimes people incorrectly refer to the computer as a hard drive, processor or CPU. The hard drive and processor / CPU are some of many internal components of a computer, but not the computer itself.
The monitor can be thought of as a dumb drawing board, all it does is display what the computer tells it to display.
Terminology: Sometimes people incorrectly refer to the monitor as a computer. The monitor can be referred to as a monitor or a display, but should not be confused with the computer.
Keyboard and Mouse
The keyboard and mouse are input devices that send commands to the computer. Basically, we use them to tell the computer what to do.
There are countless other devices, components, and accessories that plug into the computer: microphones, speakers, cameras, printers, external hard drives, etc. Most of these devices can be thought of as inputs, outputs, or both. For example, a printer can print (output) and scan (input) data.
Terminology: Devices that are not part of the primary functionality of the computer are called peripherals.
Other types of computers
Laptops, smartphones, netbooks all look different than a desktop but operate on the same basic principles. They all have a computer and some set of input / output components – a display, a keyboard etc, only in smaller devices that are more mashed together due to size constraints.