The computer has a central board called the motherboard. The motherboard is a printed circuit board with all of the components attached to it. Every other compoent links to the motherboard, either attached to it directly or via some kind or cable. The hard disk drive and DVD drive for example are usually connected via a ribbon cable. There are also usually slots on the motherboard into which you can plug other printed circult boards. These slots are called expansion slots. The sound card and the graphics card, responsible for picture or and audio fit into these slots, however there are other types of plugin cards. On the motherboard there are two special chips called the Northbridge and the Southbridge and these chips coordinate a lot of the the internal communications between the seperate parts of the computer as well as other external things like your keyboard, mouse and USB ports. http://en.wikipedia.org/wiki/Motherbo