RPM = Rate of fan spinning. Higher the RPM, the higher the airflow and noise.
CFM = A general measurement of airflow. The higher, the better the cooling, in theory.
Static Pressure = the amount of CFM that penetrates heatsinks or radiator fins. The higher the better performance for heatsinks.
Size = Thicker fans tend to have better static pressure. I recommend a good 120mmx38mm for heatsinks.
DBA = A general noise measurement. The lower the better.
Quality = The better the quality, the more accurate the above ratings, and the longer lasting the fan.
Bearing type = This determines a lot. Here are some common ones.
-Sleeve. These are very quiet and inexpensive. However, their life span is pretty low depending on the quality. Some low quality ones have problems being mounted horizontally.
-Ball bearing. These are some of the most long lasting fans you can buy. They are also loud.
-Fluid. There are a lot of fluid bearings, such as hydro-ball bearings. These are quieter than ball bearing and longer lasting than sleeve. They tend to be unreliable with their airflow and RPMs.
-Proprietary. These are getting very common. They are odd designs made by companies. Just research them before you listen to the hype.
Brands I recommend: Yate Loon, for their quality, low-cost, and accurate ratings. Antec, they make good quality, sturdy fans. Scythe, they also make good fans. San Ace, these are extremely high airflow and noise fans that are unbeatable for heatsink cooling.
I personally like quiet fans and I general rank the various specs as follows from most important to least important:
1. Size => 120mm only thank you very much
2. dBA => quiet is king; 120mm needs to spin slow than smaller fans to move the same amount of air
3. CFM => the more the better, but limited by #2
4. RPM => the lower the better, generally because of less noise, but need enough of #3 to properly cool the case
I like Yate Loon fans because the are inexpensive and pretty quiet for the price you pay. I use voltage regulators to limit the voltage to 7v instead of 12v. In my opinion, that offers a good balance between CFM and dBA (or noise). I also use Scythe low RPM fans as well.
Nexus also sells good, quiet fans with decent CFM. Last time I checked, and it has been a while, they use cherry-picked Yate Loon fans.