A heat sink is a passive device used to lower the operating temperature of high-current electronics. The main processor on the computer you are viewing this article on is attached to a heat sink. They are used in regards to radio control on high-current electronic speed controls and motors.
Made of extruded and/or machined aluminum, a heat sink works much like a radiator by greatly increasing the surface area of the component in question within a confined space.
Heat sinks are generally seen clamped on the drive transistors on older electronic speed controls. Advances in technology have eliminated the need for a heat sink on all but the largest, most powerful controllers. Often, heat sink compound is applied to the component and heat sink before mating. Heat sink compound is a thermo-conductive grease that draws more heat from the component before transfering it to the heat sink.
The E-flite Blade CP micro-R/C helicopter requires the use of heat sinks and compound on both drive motors when used in conjunction with that aircraft's aerobatic upgrade kit.