A printed circuit board or pcb, is a plate or pcb board used for placing the different elements that conform an electrical circuit that contains the electrical interconnections between them.
The most simple printed circuit boards are the ones that contains copper tracks or interconnects pcb board only on one of its surfaces. These kinds of boards are known pcb board as 1 layer printed circuit board or 1 layer PCB.
The most common PCB's manufactured pcb board today are the ones that contain 2 layers, that is, you can find interconnects in both surfaces of the board. However, depending on the physical pcb board complexity of the design ( PCB layout ), the boards can be manufactured of 8 or more layers pcb board.
