Well there are 2 types. One is 3 pin and another is 4 pin.
The two pins (power and ground) can go by themselves (as a result you can make a usb case fan)
The third pin is for RPM measurement. All computer fans have it these days.
And the fourth pin is for PWM fan control. This is used to let the motherboard control the fans based on temperature, etc.
For the last one, I'll let somebody answer.