If you opened up the PC case (not the monitor) the largest circuit board (usually covers one whole side of the box) is the motherboard. It is called a motherboard because all of the other components either plug into it, or plug into cards which are themselves plugged into the motherboard.
As is it one of the largest components. Replacing it is not a trivial thing. It is fairly expensive and careful choices have to be made to ensure that your other parts would still work if you just changed the motherboard.
Normally you would only change the motherboard if you were upgrading your PC and had decided to change it, and may be the main processor chip, the amount of memory available and possibly the graphics card.
Stephen, October 2005
