About Computer Heat sinks:
Computer heat sinks are vital to keep modern processors cool. There are several common chips that require heat sinks to function. These include the CPU, the Memory controller and the video processor. Some computers also have heat sinks on the RAM and other chips on the motherboard (usually overclocking setups).
The goal of a heat sink is to dissipate the maximum amount of heat. This is accomplished by attaching a thermal conductivity material to the heat source, then increasing the surface area of the piece of metal. A fan is sometimes placed over the heat sink (an active heat sink) to better transfer the heat from the metal to the surrounding air.
A good heat sink will usually be quite large and have many baffles. The number of baffles increases the surface area, allowing greater heat dissipation. Most heat sinks available will be made from aluminum. These are cheap and usually work fine, but if you plan on overclocking, you will probably want a copper heat sink. Copper is much more thermally conductive than aluminum, although it is rather expensive. This increase in thermal conductivity allows the heat to flow from the heat source into the baffles faster, and thus be dissipated faster and more efficiently (i.e. a copper heatsink can dissipate more heat than an aluminum one of the same size).
The way a heat sink attaches is dependent on what it is attached to. Some heat sinks use a thermal epoxy (a heat conductive glue) to place the heat sink on the chip while other applications use a thermal paste (a thermally conductive paste that goes between the processor and the heat source) and a clip to securely press the heat sink down. Heat sinks on CPU's are usually replaceable if you decide you want to overclock, or if your CPU keeps getting hot. Most other heat sinks are proprietary, or no one makes replacements.
And remember, no matter what type of system you are running, it needs a heat sink on the processor at least. The chip will fry itself in a matter of minutes (less if it's a faster, hotter chip) without one.
Thermal conductivities of various materials can be found at the thermal conductivity node. Note the difference between copper and aluminum, and how high diamond is. For heat sinks, you want a highly thermally conductive material, thus higher numbers are better.