All materials on Earth are either conductors, semiconductors, or insulators. A conductor allows electricity to flow through it easily. Metals are all conductors. Gold is the best conductor, with very low internal resistance. It is impractical to make wires out of gold, however, because of its cost. Lead is the worst conductor, with very high internal resistance. Good balances between the two extremes are copper and aluminum. Copper is a good conductor, and relatively inexpensive and plentiful. Aluminum is also a good conductor of electricity, but not as structurally strong as copper. Furthermore, aluminum is prone to corrosion, and its longevity is not as good as copper.
Insulators block the flow of electricity. Some examples of insulators are air, rocks, wood, rubber, and most plastics. Wires usually have a plastic insulated coating, to prevent electricity from flowing into other wires. Insulators are also used to make circuit boards. A circuit board is a flat piece of insulating plastic, about as thick as a poster-board. It has flat copper wires glued on one side, to conduct electricity. On the other side, components are attached to the wires by holes drilled through the board.
Semiconductors are neither conductors nor insulators. The blockage of electrical flow is called resistance. Conductors have very low resistance. Insulators have very high or almost infinite resistance. Semiconductors are halfway in between. Carbon is a common semiconductor, and used often to make electronic components called resistors. Resistors drop voltage by absorbing electricity. For example, electricity out of a car battery is 12 volts. A plug-in cell phone charger might need only 9 volts at the output. The electricity may be reduced down by resistors inside the transformer case, which drops the electricity down from 12 volts to 9 volts. The resistor absorbs the extra 3 volts.
Switches act like like breaks in the wires, which stop electricity. Unlike an accidental break, a switch can make or break internal contacts, causing the wire to conduct or non-conduct, depending on the position of the lever. Many forms of switches exist, from simple on-off switches to rotary switches, which make or break many contacts. An example of an "on-off" switch is a light switch on the wall, which just controls one light. An example of a rotary switch is a "Low-Medium-High" switch in a fan. A special kind of switch is called a 3-way switch. Two 3-way switches allows you to control one light from one of two areas in the house.
In order for electricity to be functional, it has to be used somewhere. The load is at the end of the line, and it is where the electricity is converted to useful work. A lamp is an example of a load. Wires carry the electricity from the grid into the house. The wires are insulated, so if they touch, the electricity does not jump from wire to wire causing a short circuit. From the wire, electricity is controlled by the wall switch. Finally, it reaches the load, which is the light. Thousands of different types of loads exist in the world. Another load is the refrigerator. Electricity flows into the appliance. Inside the refrigerator are the compressor motor and a light. An automatic temperature switch turns the compressor motor on and off. When you open the door, a switch near the door is tripped, turning on the interior light.