The most basic type of PCB is the original single-layer board which, as the name suggests, involves mounting one layer of copper on a substrate. Later developments led to double-layer and multi-layer PCBs, which saw multiple copper layers applied to two or more substrate layers. This made it possible to add a much higher density of components to the PCB and use them to create more powerful devices. http://www.betonpcb.com/high-frequency-pcb/