Transistors

A transistor with gate, drain, and source terminals is known as a field-effect transistor (FET). FETs control the flow of current through a semiconductor by applying a voltage to the gate, which alters the conductivity between the drain and source. Wikipedia autodesk.com

in our case, we have an irf520 transistor and the pinout is like

1: Gate - How we control the flow of power using the arduino

2: Drain - Where we plug the fan into to receive the 12v power

3: Source - Where we retrieve the power, probably a battery or power supply

PC Fan Pinout

   Green: Sense. Used to measure RPM
   Yellow: +5V, +12V or +24V (depends on fan model, usually 12V for desktops, 5V for laptops)
   Blue: PWM control signal at 5V, 25kHz
   Black: Ground