Although a computer generally consists of several different parts, each with its own specific set of tasks to perform, a desktop computer can be broken down into three main parts: the monitor, the system unit, and the keyboard, all of which are essential to any functional desktop computer.
The system unit of the computer contains the hard drive, disc drive, CPU, graphics card, sound card, and countless ports used to connect external devices to your computer. In essence, the system unit is "brains" of the computer. Without it, you couldn't store data, run processes, or do much of anything at all.
Your computer's monitor lets you see what you're doing. Even if you have everything needed to start your computer, you can't do anything without the monitor. Connecting your monitor to your computer's system unit lets you view your computer's desktop and see just what you're working on. Computer monitors look a lot like television sets, and with the development of high-definition technology, the gap of differences between the two is closing fast.
The keyboard is essential to any desktop computer. It lets you communicate with your PC. While a computer mouse or a trackball lets you control your computer's performance and tell it what to do, typing in a web address, writing a paper, or entering text onto a screen using a mouse or trackball is tedious and cumbersome, and neither peripheral is necessary to work on a computer.
Other Important Parts
Printers and scanners allow you to create and send physical documents to and from your computer. Speakers let you hear sounds produced by your computer. Touch-screen technology is a growing trend, combining the features of a keyboard and monitor into a single device. Microphones, headsets, and webcams are also important, but aren't essential to the computer's basic functions.