A guitar pickup is basically the same in construction as many bass pickups and use most of the same principles. A magnet is normally permanently mounted inside a pickup and is used to magnetize the ferrous string. As the strings vibrate when being plucked an induced voltage and impedance is created that travels through the controls of desired specifications and amplified at the amp. The magnetic path is determined by the ferrous material or pole configuration, bobbin shape, magnet wire gauge, number of turns and the magnetic strength in proximity to the strings. If you need to understand the technical aspects of the relationship between the coils and magnetic fields than I would suggest taking a course in Physics and much of this would be better understood.