Electrical resistance is defined as the resistance to flow of electricity through a material. Even the best conductors, such as gold, have some resistance. Resistance elements essentially fall somewhere between a conductor and an insulator. Resistance can also be considered a measurement of how tightly a material holds onto its electrons. For example, common resistance elements in a circuit are lights, motors, and electrical resistance heaters.
The electrical resistance of a material is measured in units called "ohms". The lower the resistance of a material, the better the material acts as a conductor. For example, copper has a lower electrical resistance than aluminum; copper is a better conductor. The resistance value for most materials is listed in physics or science books.
We can use a water piping system as an analogy. The resistance in the water pipe to the flow of water comes mainly from the size of the pipe. Rust and corrosion inside the pipe, objects stuck inside the pipe, and the number of bends and fittings all add up to increase the resistance to the flow of water.
The same is true of current flow in an electric circuit. A number of factors determine the resistance to current flow such as wire diameter, wire length and any impurities in the wire's makeup. For example, smaller wires have more resistance than larger diameter wires and longer wires have more resistance than shorter wires.
When electricity flows through any resistance, energy is dissipated in the form of heat. If the heat becomes intense enough, the conductor resistor may actually glow. This is exactly how an incandescent light bulb works. The filament is made of a material that will resist the current enough to heat up and glow.
The scientific symbol for electrical resistance, which is measured in ohms, is the Greek letter Omega. Electricians and practical wiring books typically use an "R" to represent resistance. So in this title, we will use the practical symbol "R" to represent resistance in ohms.