All electronic components generate heat during their operation. This heat increases the temperature of the components and if not dissipated at a proper rate will burn the component or reduce substantially the component's service life.
To remove the generated heat, air is generally used. The heat dissipation rate depends on air velocity, surface area in contact with the flowing air and temperature difference between air and the hot surface in contact with the air. In many cases the surface area of the component in contact with the air is too small to remove the generated heat at the permitted temperature difference between the component surface and the air. To increase the surface area in contact with the flowing air a heat sink is used. The heat sink is usually made from aluminum or copper and is composed of a flat base which is thermally attached to the top of the component absorbing the heat generated by the component. From the base protrude large number of pins or continuous fins comp[rising large surface area in contact with the air. The fins or pins absorbs the heat from the base and transfer the heat to the cooling air flowing among the fins.
Look what happened when the CPU COOLER is removed
In most cases the air flow is generated by a fan or fans which are either remotely disposed in respect to the heat sink, such as on the wall of the enclosure while in other cases such as with CPU COOLERS the fan is mounted on the heat sink above the fins blowing the cooling air directly on the fins and base.
The heat sink is characterized by its thermal resistance which is defined as the amount of heat the heat sink can dissipate at a given temperature difference between the average temperature of the base and the temperature air flowing into the heat sink. The thermal resistance decreases exponentially as a function of air velocity, between zero air velocity = natural convection to 3 m /sec, after which the thermal resistance remains unchanged.
Heat sinks are referred to also as heat dissipaters,