According to Ohm's Law, voltage and current are directly related - a change in voltage produces a change in amperage. Ohm constructed wires of many different lengths and thicknesses. Today, we simply call these devices resistors because all they do is resist the flow of current. He discovered that, for each resistor, no matter what potential difference he applied, the resistance was the same. Electrical resistance is another term for the friction of material to the flow of the current through it.
Resistance is anything that causes an opposition to the flow of electricity in a circuit.
To force current will generate heat in the conducting material. Resistors are very important in circuits. They are used to control current or potential difference in a circuit. All materials have resistance, whether they carry current or not. The resistance of a material is always the same - unless it is physically altered.