There are two types of 12VDC PC fans of various sizes:
3-pin and
4-pin.
The 3-pin connector has +12 VDC, GND, and a +5V tachometer output (middle pin) with usually two pulses per revolution. The +12 VDC is commonly reduced using diodes (e.g. Noctua), a voltage divider using a potentiometer (extremely common), or filtered PWM (most digital fan controllers), to reduce the fan speed. Intelligent controllers use a higher starting voltage, to ensure the fan starts rotating, as the minimum voltage needed to start the fan varies; and essentially control the voltage to get the desired RPM (which can depend on the monitored temperature in more complex fan controllers).
The 4-pin connector has +12 VDC, GND, open drain tachometer output (controller expected to have a pull-up to 12VDC) with two pulses per revolution, and a 5V PWM control input at 25 kHz (21-28 kHz, less than 5mA). For these, you use the duty cycle of the PWM control input to determine the speed.
I don't like the 4-pin ones, since I tend to "hear" the ultrasonic PWM whine, and prefer the 3-pin ones instead. I've used e.g. Lamptronic FC5 V2 (I think), that controls four separate 3-pin fans to individually set RPMs (and if you use the same type of fans, you can put them in parallel, with only one providing the tach output), and has four thermocouples with Fahrenheit/Celsius display. It's very useful when optimizing airflow and cooling for custom silent cases.
The widest variety of PC fans is currently in the 120mm×120mm×25mm category. You have low-RPM quiet fans, high-RPM "hairdryer" fans (i.e., lots of airflow, but also a lot of noise), fans designed to work against a pressure differential (filters, restricted airflow), fans designed for unrestricted airflow, and so on. At one point, I probably had three dozen different fans I'd tested, although only used about four of them in case builds... The differences between them are surprisingly large, and it is very common that dropping the voltage (for the 3-pin fans) to 10-11 volts gives a significant reduction in the noise, with a comparatively smaller change in airflow.
Because larger fans need smaller RPMs for the same airflow, you should try and use the largest fan you can, to get the best airflow while keeping the noise to a minimum. Baffles and filters are also very useful.