Static pressure

Ok, here we go...

Every fan has a performance curve which describes it's flow and pressure characteristics. The higher the pressure across the fan, the lower the flow through the fan and vice-versa. The pressure across the fan, and hence the flow through the fan, is dictated by the system in which the fan is installed. Things that impede airflow, such as small holes, heatsinks, filters, wires, convoluted flow paths, etc. cause an increase in the pressure required to drive a particular flow. The effect of this is that the more resistance you put in the path of the fan, the more the flow through the fan will drop.
As for the the static pressure quoted by manufactuers- I assume this is either the max pressure the fan can deliver (ie. when flow = 0), or the pressure at the fan's design point (ie. flow = x cfm at y mm H20- or whatever other units of flow and pressure you want to use).
Of course, manufacturers don't seem to make performance curves available to the public, and trying to work out the airflow resistance of your case or heatsink is nigh on impossible, not to mention a waste of time. What you need to know is that systems in which there is a high resistance to flow (heatsinks being a common example) will require a higher static pressure rating than systems which have a low resistance to flow (typically cases). So if you have a high resistance system, like a CPU heatsink, installing a powerful fan with a high pressure rating will ensure you get a decent airflow flow over the heatsink.

Hope that helps...