The PCB is the actual board whereupon all of the components will be mounted. The PCB is actually several layers of flat resin which have the various circuit lines, called 'traces', embedded into them. A typical PCB will have four layers, with the top and bottom being the signal layers. The two middle layers will be used as the ground and power planes (see Figure 1). By placing the power and ground planes in the center, fixes and corrections to the signal circuitry is more easily accomplished.

Figure 1

Some motherboards will require six layers, such as those designed for dual processors or when the processor pin count approaches 425 pins. This is because the signal traces must be kept far enough apart to prevent crosstalk and the additional layers allow for this. Six layer boards may have three or four signal layers, one ground plane and one or two power planes. With newer chipsets, three signal layers and two ground planes are typical to accommodate the additional power requirements (see Figure 2)

Figure 2