Amateur Radio Manual/What is Resistance and Conductance
Unlike an ideal conductor the real material constituting any kind of conductor, e.g. copper, contains in some amount impurities (other materials). Own characteristic qualities to resist to the flow of charges (electrons) if a difference of potential is applied (voltage) can be measured too. Temperature influences heavily this characteristic of resisting to the charges flow. We can imagine that inside the conductor the atoms and electrons become exagitated because of temperature. Higher temperature inducts more exagitation and electrons move in all directions increasing again the resistance of the conductor.
In other words, resistance is a phenomena which counterposes to the ordinate flow of charges in a conductor.
Resistance is measured in Ohm (Ω) (from the German scientist).