Diffrentiate between active heatsink and passive heatsink?

In a active heatsink system you have both a heatsink and a fan thus for the most part an active heatsink cools better with one exception. Should the fan fail the active heatsink will quickly overheat.

Passive heatsinks relay on transferring heat without the aid of a fan. The advantage here is that a passive heatsink is fail proof and uses 0 electricity, however passive heatsinks don't typical cool as well.

Passive heatsinks work great for supporting chips and RAM cooling. They are also found in servers because of the no fail aspect.

EnzoTech produces some passive heatsinks you can read a bit more about them here:

http://electricalninja.com/