Heat pipes are one of those weird things that you don’t realize are all around you until you start looking. It is a copper tube with a near vacuum in it. It also has some liquid in it (usually water or ammonia). The idea is that if the bottom part of the pipe gets hot, the liquid evaporates and the gas rises to the top of the pipe. Then you cool off the top of the pipe and the liquid inside condenses on the inside of the tube and runs down the side of the tube all over. It is like a refrigerator but without any moving parts.
Heat pipes are used in heat sinks that cool off computer processors. They are also surrounded by fins and a fan that help cool the top part, so even if you open up your computer you may not see them. I learned about these this weekend helping friends out with their computer which was overheating. I am still not exactly sure what was going wrong, whether the liquid had escaped the tube or the tube had somehow deformed and was no long making contact with the slab of copper that goes on top of the processor (or if something software-related was making the processor work too hard). I put in some small metallic disks (dimes) to shim the tubes down into making contact with the copper slab and got some thermal compound to put around them as well and hope that will work. But if the liquid has escaped then it will never work. You could also buy a new CPU cooler but this particular computer is very compact and it is hard to find coolers this small. It only has two L-shaped heat pipes in the heat sink. I looked inside my Dell and it has 3 U-shaped heat pipes with a bigger fan. Wikipedia says a heat pipe is more efficient at moving heat than a solid piece of copper.