Pickups are the strips you see under the strings on an electric guitar. They turn the physical vibration of the strings into an electrical signal. On an acoustic guitar, you hear the sound waves created by the strings’ vibration. On an electric guitar, you hear the electrical signal after it’s been run through an amplifier and comes out of a speaker. (Although the Clear Science art director couldn’t have an amp when he was a kid, so would put his ear on the horn of the guitar and hear the sound waves traveling through the solid guitar body.)Guitar pickups work by electromagnetic induction, so let’s talk about what that is. If you have a coil of wire (a copper coil is illustrated above), a magnet passing through the coil will induce an electromotive force (EMF) in the coil. This EMF will cause an electric current to flow. (Incidentally, this is also how electric generators work.)
This concept is called Faraday’s law of induction, discovered by Michael Faraday. The EMF is equal to the time rate of change of the magnetic flux through the coil. Magnetic flux means the amount of magnetic field per area per time. Said plainer, this means changes in the magnetic field will cause some kind of current.
via: http://clearscience.tumblr.com/post/3564812865/pickups-are-the-strips-you-see-under-the-strings
