Fans 3pins and 4pins

Can someone please explain me a bit about 3-pins and 4-pins Fans? Yes, I admit it's a stupid question, but I'd like to know a bit more about them. Generally, when you want to make a fan spin, you'd need 2 connections: one plus and one minus (or ground). Now if you make the plus adjustable (maybe even with a potentiometer) you can adjust the speed. So far so good. Of course you might want to check the rotation/minute (RPM), so I can imagine having another pin that tells me that info. Is that what a 3-pin does? Why would you need a 4-pin? I think PWM stands for Pulse Width Modulation. But still I don't see the need for 4-pins? And maybe the more practical side: I understand that you can connect a 3-pin Fan to a 4-pin header (is that correct?), but can you also connect a 4-pin PWM Fan to a 3-pin header?

And while we're at it: I'm looking to build a Phenom2 x4 945 build with a XFX4850 512 video card and a OCZ Fatal1ty PSU. Any suggestions about case-fans and how much airflow they should have?

Thanks

The wires in a 3 pin fan consist of: power, ground, and rpm sensing.
the wires in a 4 pin fan consist of: power, ground, rpm sensing and PWM (for RPM control)
But the 4 pin connector is generally just for the CPU.

You can control the RPM of a 3 pin fan too, by using fan controllers and some motherboards do it.
And the fans you buy for cases are all 3pin or molex.

what case do u have?

yahh, the forth pin is PWN which will adjust the CPU fan speed according to its temperature. im pretty sure its more of a CPU thing though. (its a nice thing to have btw)