A resistor is a two-terminal electronic component that produces a voltage across its(resistors) terminals that is proportional to the electric current through it in accordance with Ohm's law: V = IR.
A resistor is coated with paint or enamel, or covered in molded plastic to protect it. Because resistors are often too small to be written on, a standardized color-coding system is used to identify them. The first three colors represent ohm value, and a fourth indicates the tolerance, or how close by percentage the resistor is to its ohm value. This is important for two reasons: the nature of resistor construction is imprecise, and if used above its maximum current, the value of the resistor can alter or the unit itself can burn up.
Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size, and position of leads (or terminals) are relevant to equipment designers resistors must be physically large enough not to overheat when dissipating their power.
A fixed resistor has a predetermined amount of resistance to current, while a variable resistor can be adjusted to give different levels of resistance. Variable resistors are also called potentiometers and are commonly used as volume controls on audio devices.
A rheostat is a variable resistor made specifically for use with high currents. There are also metal-oxide varistors, which change their resistance in response to a rise in voltage; thermistors, which either raise or lower resistance when temperature rises or drops; and light-sensitive resistors.