Circuit boards are used to hold electronic components and to provide a mechanical means of holding them on a space and to connect them together electrically. This is a printed circuit board with different resistors, capacitors, and active circuits in these black packages. The circuit board acts to hold them all together as a collection, and to connect them electrically.
Now the circuit board is made of several layers. The first layer that you see is this green layer. This green layer is made out of a polymer, called LPI, which is a photo image-able polymer that goes down as a liquid and dries in forms of solid layers. Now this green layer is necessary to prevent the solder from crossing over and shorting out traces that should not be connected. Just beneath that layer is a layer of copper that provides one layer of conductors which connect the circuit electrically.
Now also on the back you have another green layer. In this case, another layer of copper. This is called a two-layer circuit board. Holding the layers apart is an insulating layer known as FR4. FR4 is fiberglass, which is glass fibers laminated together with epoxy resin. The nice thing about fiberglass is that it has a very high operation temperature. It doesn't burn until it hits 300 or 400 degrees Fahrenheit, and it also makes an excellent insulator. FR4 is the most common material for printed circuit boards.
Some printed circuit boards also have an area made to connect to external circuits. In this case we have an edge connector, which has gold plated fingers, which are designed to connect with a connector and attached to other parts of the circuit.
Now FR4 circuit boards are rigid. They don't bend very much. Alternative circuit board materials, such as are available today, which allow for one to make a flexible circuit.
Here's what a circuit board looks like without electronic components. As you can see we have the green layers. You can see the outline of the copper underneath. And you can also see this white layer. This white layer is known as silkscreen, and is useful for assemblers to know where to put the different components. In this case, we have labels for resistors, with the letter R, labels for capacitors with the letter C, and labels for the active circuits with the letter U.
These designators allow an assembler to choose the proper component and put them into the circuit board in its proper orientation.