A PCB is a printed circuit board, also known as a printed wiring board. It is used in electronics to build electronic devices. A PCB serves two purposes in the construction of an electronic device; it is a place to mount the components and it provides the means of electrical connection between the components.
A PCB starts out as a thin, non-conducting sheet of material. The most common material used is a glass fiber epoxy laminate material. A thin layer of copper is then chemically deposited on each side of this material.
The next step is to "print" the connection diagram onto the PCB. The connection diagram is the wiring required to connect the components. In the very early days of electronics, these connections were in fact done with wires. This is the reason PCBs are also sometimes referred to as printed wiring boards. The "printing" is usually done by photographically transferring the image to the board. This image is "printed" with an acid resistant material.
Next, the PCB is put into an acid bath. The acid bath removes the copper from the board, excepting the areas protected by the resistant material. This process leaves the connections or wiring "printed" on the PCB. Next, holes are drilled in the board to allow the components to be mounted to the board and the PCB itself to be mounted to the case protecting the electronics. Finally, a protective coating is applied to the board to prevent corrosion of the copper traces.
The above process describes what is referred to as a doubled sided or two layer PCB. It is possible to make them of almost any number of layers by repeating the process above and laminating the resulting boards into a single PCB. The circuit board inside of the computer on which you are reading this document has a minimum four layers and is more likely to be an eight or even 12 layer PCB.
It is also more common now for parts to be soldered directly to the board, eliminating the need for holes into which leads (wires) from the components must be inserted. A component soldered to the board is logically called a surface mount component, and the board is a surface mount PCB. This technique allows a large increase in the number of components to be mounted on the same PCB, or the same number of components to be mounted on a much smaller one. However, surface mounting also requires better quality control on the manufacture of the PCB.
A PCB is now used in almost every electronic product available. It is by far the most common method currently used to connect and assemble the components of almost any electronic device.