This page explains the differences between their three main design groups (F, P and S)
http://noctua.at/en/which_fan_is_right_for_me
There are other important factors, some summarized in their main product table
http://noctua.at/en/products/fan
Note the suffixes PWM for the newer 4-pin PWM fan design, and FLX for the older 3-pin design. These two have a difference in the method that the mobo header must use to control the fan speed, so it is important to match fan design with header capability. The suffix "ULN" on some fans is just a slower fan that makes less noise, but thus also supplies less air flow.
There are two other main groups, too. The "redux" line is their older designs (and a different color scheme) in a limited range of models that are slightly cheaper to buy. Then the IndustrialPPC are higher-power fans delivering more airflow against higher back-pressures, but using more power and generating more noise at maximum speed.
Finally, note that a few of the first main NF-A... group of 120mm fans come in thinner (15 mm thickness) versions, whereas common fans are 25 mm (1 inch) thick.
Many Noctua fans come with "Low Noise Adapters" you can insert into the fan connectors. These really are to be used only when you connect them directly to a fixed 12 VDC power supply (such as from a PSU) and need to reduce their voltage and speed to reduce cooling airflow and noise. If you are using the fans with some other means of speed control - either using the mobo automatic control features, or using a third-party fan controller - I recommend you do NOT use those "LNA" units. They will just limit the fan's maximum cooling ability and interfere with the controller you are trying to use.