Electric guitar pickups convert string vibration into an electric signal. They consist of a magnet wrapped in fine copper wire to create a magnetic field, which causes the string above it to do the same. When the string is plucked, the movement of its field induces a current in the pickup coil, thereby electrifying its sound.
Single-coil electric guitar pickups have one magnet and wire wrap, while humbuckers have two. The second coil allows humbuckers to cancel out interference, 'bucking' the hum of early single-coils. These days, noiseless single-coil pickups exist, along with a kaleidoscope of different pickup sizes, tone profiles, and output levels to match almost any guitar.