I've been browsing on the mwave website (Australia Computer hardware) and stumble across these two 120mm arctic case fans.
Im not too sure what are the differences between the two as their model numbers are almost identical and they look the same. However, the prices are different. The links are attached below.
Im looking to buy 4 of them for my haf 922 case as 2x 120mm top exhaust, rear exhaust and bottom intake.
The more expensive one is a PWM (pulse width-modulation) fan. It has 4 pins for connection and is meant for the mobo to adjust the speed using the 4th pin (PWM control). It CAN work on a 3 pin fan header, but you will have paid for the feature and neglected it.
The less expensive one is a 3 pin, voltage in/ground/speed sense fan, without the PWM control pin. It is cheaper to make.
Look at your motherboard and see what kind of headers for fans you have. Typically most mobos have at LEAST 1 4-pin PWM capable header for the CPU fan, and some mobos have 1 more 4-pin for the rear case fan. The remaining headers are usually 3-pins. So check your particular board and see what you have. If you are only planning on using these for case cooling and/or you have a fan controller (without PWM, check the pin count), buy the cheaper 3-pin variety.