Resistance is a property that hinders the flow of electricity and converts it to another form of energy like heat. It is measured in ohms and depends on four factors - the type of material, its cross-sectional area, length, and temperature. Ohm's law states that the voltage between two points is directly proportional to the current flowing through, and resistance can be calculated by dividing the voltage by the current. An ohmmeter is used to measure resistance in a circuit.