Motor itself is a brushless, polyphase permanent magnet (ring in fan hub), AC motor with DC-AC inverter built in. Up to 12vDC is supplied thru red wire and negative for it is the black one next to it.
Usually blue wire, sends tachometer signal to MB so you can see RPM. PWM fans/motors also have a simple transistor switch inside which turns off power to motor in various lengths and frequency from controller on the MB. Those pulses are adjusted according to tachometer reading to keep fan working at desired speed. For such motor/fan it's enough to just supply 12vDC power to red and black wire and it will work at it's full rated speed which is also determined by inverter's frequency. If you lower the voltage (by resistor for instance) it will loose some power and spin slower.