Bi li moglo ovo:
It uses the parallel port to send a digital (either on, or off) signal to our circuit. This signal is fed into the base of a transistor, through a protective resistor. When the transistors base goes above a certain level (usually about 0.7v) it alows current to flow between its other two legs, its emitter and collector, which are capable of providing the high power needed to drive a fan or cold-cathode. It's acting as a kind of electronic switch.
The program is able to manipulate the signals from the parallel port in order to switch the transistors, and thus our fans on and off. Take alook at the diagram: