Magnetic pickups are generally devided into two types, Single-coil and Humbucking. They are made of copper wire spun around a magnetic coil(s). Pickups are sensitive to electronic noise, emitted from everything from light bulbs to computer screenss. This noise is quite annoying, and can be heard as a faint hum.

Humbuckers amplify less of this "hum", because they consists of two standard magnetic pickups (single coil) wired out of phase. Common-mode signals (i.e. signals that radiate into both coils with equal amplitude) tend to cancel each other out when they travel through both coils.