Wikipedia gives the official definition of a PCB as:
A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.
Most PCBs are made in factories with special equipment which can produce large batches of PCBs in sheets or 'panels'. The video below takes an in-depth tour of a PCB manufacturing facility:
There's a number of different ways to make your own printed circuit board. Each method starts with a "copper-clad" board - a flat piece of rigid material covered on each side with a thin layer of copper.
CNC Milling machines are great at making PCBs. They use a rotating motor controlled by a computer to cut away unwanted copper, leaving only the traces needed by your circuit.
You can also make PCBs with a chemical process called etching. First, the area we don't want removed from the board is covered in w a protective layer that will resist etching. Next the board is submerged into a chemical that dissolves copper. Once the etching process is complete, the protective layer is removed, leaving only circuit traces.