A fanless CPU cooler basically works from heatpipe technology alone.
Theres a gas inside the pipes that lifts hot air up, and out at the top basically, and the process just repeats and repeats through a natural process.
They tend to not need any fans, due to a large amount of heatpipes on the cooler itself, to get rid of enough heat.
Many say though, that if you are to do any overclocking, place a low speed fan on as well.