also known as PCBs.
The standard way of packaging an electronic circuit
Each electronic circuit consists of components and wires interconnecting these components.
Printed circuit boards, consist of a plastic board, with a thin copper layer. Through photographic methods, certain parts of the copper layer are protected by an acid-resistant layer. Then the whole board is exposed to an acid, which removes all the unprotected areas. Left is a grid of copper lines, which forms the wires of the electronic circuit.
Electronic components are then placed onto the board by soldering their metal contacts to the appropriate copper areas, thus forming a complete electronic circuit. Mounting the components onto the board is done with SMD or through-hole technology.
Printed circuit boards have been in use since the middle of the 20th century. Before that people used free floating wires which where soldered together, forming something resembling a complicated spider-web. Obviously this was a very bad method, often resulting in unwanted errors and a bad physical stability of electronic devices.