When a potential differenceV is applied across the ends of a wire a steady current I quickly begins to flow. In different materials the amount of current that flows will be different even if the voltage is the same.
The scattering of charge carriers off impurities, imperfections, and vibrations of the atomic lattice creates the resistance to the flow of charges in the wire.
For a fixed potential, the resistance R determines how much current will flow.
Definition:
R = Electrical Resistance (SI: Ohms = W = V/A) V = Voltage across the ends of the wire (SI: Volts = V) I = Current in the wire (SI: Amp = A)
Resistors are used in a circuit to control the current flow or the voltage at different locations in a circuit.
OHM'S LAW: V = I R
For many conducting materials the current flow is directly proportional to the voltage applied or - equivalently - the resistance is constant over a wide rage of applied voltages.
Material which do not obey Ohm's Law will have a different resistances when different voltages are applied. This often happens because the resistance of many materials is temperature dependent and when power is dissipated in the resistor due to current flow, the resistor heats up.
