Thermal compound
Alternatively referred to as
heat dope, heat sink jelly, heat sink compound, thermal gunk, and silicon compound, thermal compound is a sticky paste that is placed directly onto the CPU. Allowing for a more direct heat transfer between the CPU and heat sink and preventing air gaps from forming between the CPU and heat sink. This compound is an inexpensive solution and can be found at any local Radio shack or computer repair center.
In the picture to the right, is an Arctic Silver thermal compound dispenser and an older CPU with thermal compound.
To apply the thermal compound, apply a very thin layer onto the CPU using a flat tool such as a post card, piece of paper, piece of plastic, credit card, etc. Do NOT user your bare fingers to apply the compound as they contain oils and other substances that can cause issues. If you do need to use your finger cover it with plastic or use a plastic glove.
Many OEM computer manufacturers will use graphite pads instead of a thermal compound. If you're changing heatsinks or overclocking your PC and need to apply a new layer of thermal compound make sure to remove this pad first.
Also see: Heat sink, Processor definitions