Printed circuit board
A printed circuit board (often abbreviated as PCB) is a technology used for cost-effective production of multiple instances of a module.
A thin fiberglass sheet is coated with a thin layer of copper; next, a pattern of 'resist' is applied, outlining a series of traces which will connect together the components placed on the board. (This resist is usually printed on the board, hence the name.) The board is then placed in a chemical bath which dissolves the copper where it is not protected by the resist, leaving the traces. (The term for different revisions of a card, etches, comes from this step.)
The first PCB's were single or double-sided. Later, techniques for producing so-called 'multi-layered PCBs' were deveioped; a number of thin PCBs were created, and then glued together; this allowed the production of complex circuits which could not be 'routed' with only two surfaces.
When PCBs were first used, they had holes drilled in them, where the leads from components would be inserted. Most components are now designed to be surface-mounted, so the PCB layout has pads for them to be soldered to.