Ultimately all the heat is removed from your system by air moving past the radiators fins. This air flow is create by fans.

There are three main factors effecting fan performance they are

Pressure

This is a measure of the maximum back pressure that can be achieved at zero flow. This is an important factor when trying to push the air past restrictive radiator fins. Higher static pressure numbers will allow more air to pass by the radiators fins resulting in lower water temperatures.

Flow

This is a measure of the maximum flow that can be attained when no back pressure is applied to the fan. This is normally measured in feet per minute. Higher this number the better

Noise

Unfortunately when flow or pressure is increased this generally leads to increase noise. Therefore noise tends to be the limiting factor when it comes to fans mainly due to the close proximity of the user to the fans.