this image has been linked to SOOO Much.
basicly. a heatpipe is a sealed tube. all the air is sucked out and a small amount of fluid is injected into the tube, called the working fluid.
What happens, is when you take away all the air, you create a vacumm in the tube, which creates a different atmospheric condition for the fluid inside.
the atmospheric pressure of the tubes innards is less than the atmospheric pressure outside it.
what this does, is allow the fluid that is inside the pipe, to boil at a lower temperature when heat is applied.
lets say you try to boil water.. it boils at 213º F right? if you remove some of the pressure pushing down on it, you can boil it at a lower temperature.
when you boil a fluid, it collects energy in the form of heat, which is able to move freely away from the source, as a vapour.
heat wants to equalize, so when you give it someplace that is colder to go, it will. the other end of the heapipe has a heatsink attached to it, and this lowers the temperture of the working fluid enough that it can recondense into a liquid. it is then collected into a wick, and returned to the evaperator side.
all a heatpipe does is MOVE the heat through the phase change of a chemical. it isnt as efficiant as direct metal to metal conduction, but it allows you to move heat over much greater distances than you could with a solid heatink.
it is ideal for conditions where a heatsink cant always be applied.
Thermalright makes heatpipe heatsinks that are just, heatsinks, with heatpipes to move heat from the base to the top where the heatsink is coldest.
other heatpipe applications have much longer heatpipes that move the heat to a completly different location
http://www.nordichardware.com/reviews/cooling/2003/ZalmanTNN500A/