Single sided PCBs are the most basic type of PCB. They contain only one conductive layer and as such constrict their use to simple low density designs. Accordingly, they are low cost and well suited to simple and high volume products.
2. Double-sided PCB
Double sided PCBs probably are the most common type PCB. They allow for the routing of traces around each other by jumping between a top and bottom layer by way of vias. The ability to cross paths from top to bottom greatly increases the circuit designer's flexibility in circuits designing and lends itself to greatly increased circuit densities. This type of board is relatively low cost, but only allows an intermediate level of circuit complexity and does not lend itself to electromagnetic interference reduction techniques.
3. Multilayer PCBs
Multilayer PCBs further increases the complexity and density of PCB designs by adding additional layers beyond the top and bottom layers seen in a double sided configuration. With the availability of over thirty layers in multilayer PCBs configurations, multilayer PCBs allow designers to produce very dense and highly complex designs. Quite often the extra layers in these designs are used as power planes, which both supply the circuit with power and also reduce the electromagnetic interference levels emitted by designs. Lower EMI levels are achieved by placing signal levels in between power planes. It's also of note that increasing the number of power planes in a PCB design with increase the level of thermal dissipation a PCB can provide, which is important in high power designs.
In multi-layer PCBs whole layers are almost always dedicated to Ground and Power. So we classify the layers as Signal, Power or Ground planes. Sometimes there is more than one of both Power and Ground planes, especially if the different components on the PCB require different supply voltages.