According to Ohm's Law, the ratio of voltage to current across an element is called resistance.
A resistor converts electrical energy into thermal energy and radiates it in the form of heat, measured in Joules. By itself, a resistor does not produce energy, nor can it be used to model an energy-producing circuit component, two characteristics that define resistors as a passive component.
Although the resistance of an ideal resistor will remain constant, the resistance of most real resistors does change with time. But as long as the change in resistance is miniscule, resistors still perform a vital function.
While the conversion of electrical energy into heat in resistors helps us cook our food, toast bread, and warm our homes, this effect is really something of a headache for most electrical engineers, who have goals far beyond cooking food and providing warmth.
Ohm's Law says that a large current will flow through a small resistance, and a small current will flow through a large resistance. This also implies that there will be a small voltage drop over a small resistor and a large voltage drop over a large resistor. It is obvious that resistors can be used to divide current and divide voltage, and can therefore control circuits. This is a function that modern electronics engineers need.
As products operate at ever-higher frequencies, skin effects, as well as parasitic inductance and capacitance threaten to alter resistor performance. Fortunately, within their effective frequency ranges, today's resistors still maintain a very steady impedance.