answer：

PCB  can be classified three categories, as follows:

A, single panel
Focus on the most basic of PCB, part on the one side, the conductor is focused on the other side. Because the wires in only one side, so this kind of PCB is called Single panel (Single sided). Because single panel in the design of lines, there are many restrictions (because only one side, wiring must around the path alone cannot cross between), so only use this kind of early circuit board.

Second, the double panel
To both sides of the circuit board wiring, but it took two wires, must have the appropriate circuit connection between two sides. This circuit is called the "bridge" between the guide hole (via). Guide hole is on the PCB, or coated with metal small hole, it can be connected to both sides of the wire. Because the area of the double panel a times larger than a single panel, double panel solved because wiring of the difficulty in the single panel (can guide Kong Tong to the other side), it is more suitable for use in more complex than a single panel circuit.

Third, a multilayer
Can in order to increase the area of the wiring, a multilayer wiring board using the more single or double. With a double lining, two one-way for outer layer or two double lining, two blocks of single outer layer of the printed circuit board, through the positioning system and alternate insulation adhesive materials and conductive graphics interconnection according to design requirement of printed circuit board becomes four, six layer printed circuit board, also known as multilayer printed circuit board. Layer number of the board does not mean that there are several independent wiring layer, under special circumstances will join empty layer to control board thickness, layer number usually is even, and contains the outer two layers. Most of the motherboard is 4 to 8 layer structure, but technical theory can be nearly 100 layers of PCB. Large supercomputers are used quite layers of the motherboard, but because this kind of computer can replace with many ordinary computer cluster, super sandwich plate has not been used gradually. Because each layer of PCB are closely combined, the actual number is generally not too easy to see, but if you look closely at the motherboard, can still see it.