Heatpipes are hollowed pipes (usually copper) that are either filled with a liquid with a high heat conductivity or a fine powder (again, usually copper). Both these methods are far more effective than using solid copper or aluminium to transfer the heat therefore they are used to provide a pathway for the heat to travel from the core to the large surface area of the heatsink fins.

Twin heatpipes on a Thermalright HR07

Single heatpipe on a Foxconn 8800GTX