Heat sinks are devices that dissipate excess power in a circuit off as heat. They serve as protective devices for components in circuits because the excess power could potentially overheat and destroy electronic components. The heat sink takes the excess power and converts and dissipates it as heat, so the excess power doesn't damage circuit components.
Heat sinks are used with many electronic components, but mainly those dealing with power. Being that heat sinks dissipate excess power as heat, they are used in electronic components that may have to deal with excess power. This includes voltage regulators and power transistors. These devices, dealing many times with large amounts of power, must be able to dissipate it as heat if fed too much voltage or current.
Heat sinks come in many different shapes and sizes but they all have the characteristic that they're metallic so that they can dissipate heat and have a circular