Resistors

A resistor is an electrical component designed to have an electrical resistance that is
independent of the current flowing through it. The common type of resistor is also
designed to be independent of temperature and other factors. Resistors may be fixed or
variable. Variable resistors are also called potentiometers or rheostats  
A few resistor types
Some resistors are long and thin, with the actual resisting material in the centre, and a
conducting metal leg on each end. This is called an axial package.
Resistors used in computers and other devices are typically much smaller, often in surface-
mount (Surface-mount technology) packages without leads.
Larger power resistors come in more sturdy packages designed to dissipate heat
efficiently, but they are all basically the same structure. Resistors are used as part of
electrical networks and incorporated into microelectronic semiconductor devices. The
critical measurement of a resistor is its  resistance, which serves as a ratio of voltage to
current and is measured in ohms, an SI unit. Any physical object is a kind of resistor.
Most metals are conductors, and have low resistance to the flow of electricity. The
human body, a piece of plastic, or even a vacuum has a resistance that can be measured.
Materials that have very high resistance are called insulators.