Motherboard
Now, the first thing is the motherboard. It's the basis of your computer. It's the first component installed in the system unit, and it holds all of the circuitry that ties the functions of the computer components together.
You can think of it like your car (which has many computer systems of its own). If you have a frame and tires, you've got a car (or you've got a system unit), but it won't take you very far! Now, add your engine - the motherboard - where all the systems tie in one way or another, and you've got the start of a working vehicle.