When choosing the correct fan option there are two confusing, yet very important parameters that you need to decide on. Air Flow and Static Pressure
Airflow: This is the maximum amount of air moved by a fan when there is no resistance, such as open air.
Static Pressure: This is the maximum pressure produced by a fan when this is maximum resistance and no air flow, such as in a sealed box.
Generally, for a wide open application, airflow is more important.
For a high resistance application, such as a compact space or a circuit board with many airflow blocking components, the static pressure is equally, if not more, important.
However, the most important thing to note is that these values by themselves are essentially meaningless. Both are maximum values set in ideal environments, which most practical applications will not meet. Which is why manufacturers supply a fan curve chart like the one below.
These charts show the relationships between Static Pressure and Air Flow. Using these curves will reflect a fans variability in real world applications. Static pressure is usually sated either in inches of water (H2O) or in millimeters of water (mmH2O). It is essentially a measure of the differential air pressure between the air pressure inside an application vs ambient air pressure outside of an application, which for airflow calculation purposes is is usually 0 (zero). There is an inverse relationship between airflow and static pressure. As the pressure differential rises, airflow drops.
The vertical axis labeled “Inches of Water” describes relative air pressure. The horizontal axis labeled “Airflow (CFM)” describes Airflow volume in relation to air pressure. The curve tells the user approximately how much air a fan will move for a given pressure reading. For instance - the curve meets the horizontal line at the bottom of the graph at about 290 (Cubic Feet/Minute). Look left along the bottom horizontal axis and notice that the air pressure (Inches of Water) is zero (No pressure difference at all between the inlet and outlet side of the fan). Notice that the curve meets the vertical axis at about 0.90 (Inches of Water) and that airflow is 0 (zero).
See also:
Source - Orion Fans Technical Terms
CUI Devices omniCOOL™ C, CF, V Series Fan Comparison & Life Expectancy