Printed-circuit boards (PC boards or PCBs) are literally the foundation of many of our electronic products. These copper-clad boards – often but not always using glass-epoxy FR-4 substrate, but increasing more exotic materials for the GHz+ spectrum – are both the structural support and interconnect medium we rely on the knot together active and passive components and their interconnects. While might be more accurate to call these printed wiring boards or PWBs, and there were attempts to change the standard designation about a decade ago, PWB never caught on.)
First, some historical background: The first PC boards were developed during WWII, using a ceramic substrate with conductive traces screened onto the surface; the leaded components were inserted and soldered into drilled holes in the ceramic. During the late 1970s and early 1980s, the PC board as we know it today came into production, again with through-hole components. Hard to believe but true: there were TV ads in the early days of PC boards from a major TV vendor (was it RCA? Or Motorola?) claiming the superiority of point-to-point, hand-wired "craftsmanship" in their TVs to the new-fangled PC board approach; but as transistors and ICs came into general use, hand wiring became a non-starter.
The use of PC boards with surface-mount technology (SMT) started in the 1990s, and lead pitch and track widths have been shrinking ever since, now down to a few mils (1000s of an inch). Double-sided and multilayer boards, some with over a dozen layers, added to the versatility. Ironically, many low-end consumer products, such as the PC mouse, often use low-cost phenolic as their substrate in single-sided layout with liberal use of jumpers to avoid the need for circuits on the bottom side; the boards are punched rather than drilled, which is quick and further lowers the cost.
Note that there is really nothing really "printed" about today's PC boards. Most are made using a subtractive process, where copper is chemically etched from unprotected areas; some use an additive process where copper is plated onto designated areas. There are also boards which are made using fine-dimension mechanical or laser-based milling (see the fascinating machines from LPKF, for example).