Types Of PCBs
As mentioned earlier the most basic boards have the components mounted on one side of the board and the conductor pattern on the opposite side. Since there only is a conductor pattern on one side, this type of PCB is called 'Single-sided.' This type of board has severe limitations when it comes to routing the wires in the conductor pattern (since there is only one side no wires can cross, and they have to be routed around each other), it is only used in very primitive circuits.
These types of boards have a conductor pattern on both sides of the board. Having two separate conductor patterns requires some kind of electrical connection between them. Such electrical 'bridges' are called 'vias'. A via is simply a hole in the PCB that is filled or plated with metal and touches the conductor pattern on both sides. Since the surface available for the conductor pattern is twice as large compared to a single-side board, and that wires now can cross (by routing them on opposite sides of the board), double sided PCBs are much more suited for complex circuits than the single-sided.