There are generally just two main kinds of guitar pickups. Magnetic pickups and piezoelectric pickups. The last one work with all types of strings. Magnetic pickups work only with metal strings. They are made of copper wire spun around a magnetic coil. These pickups are sensitive to electronical noise, emitted from everything from light bulbs to computer screenss. This noise is quite annoying, and can be heard as a faint hum. Humbuckers do not have this problem, because they consists of two standard magnetic pickups (singlecoil) wired out of phase. Because these coils are electrically out of phase, common-mode signals (i.e. signals, such as hum, that radiate into both coils with equal amplitude) tend to cancel each other. This also gives the humbucker a fatter tone, and the first choice in rock and heavy metal music. The original magnetic pickup, the singlecoil, is often used in country, pop and other "lighter" kinds of music where you want a more twangy tone.