Behind a computer there are lots of plugs and cables. The most important one is the one which (on a dell one) is usually on the top left hand corner. This is the one which supplies the computer electricity. To cool the electrics working inside the computer it needs a fan.
Over time, computers collect dust. It is attracted to the computer, and just stays there until you clean it away. Dusting it with a computer brush about once a month will keep it clean.
There are also cables that connect the main computer to the screen so the screen 'knows' what to show.
A Dell Pc comes in 4 different pieces: the keyboard, mouse, screen and the main computer. If you look at your computer screen (not a flat screen!) it sticks out behind. This isn’t where all that information is stored, it is where the projector is. The projector makes the screen have 'things' on it. The main computer (the big box) stores about everything that you save.