Bass guitar pickups use magnets wrapped in thin copper wire, or coil, to capture the magnetic field of your strings. First introduced on the Fender Precision Bass in 1957, these pups have since evolved into a few main variants. Single-coil pickups have an open sound with a present top end while split-coil pickups are two coils wired in series (the signal passes from one to the other).
Humbuckers, on the other hand, use two coils wired in parallel and use some audio science to eliminate electrical interference (thus 'bucking the hum'). Bass guitar pickups are either passive, with no internal power source, or active, with a battery-powered preamp that allows for added tone shaping. If you'd like a basic rundown of all things bass guitar, check out our Back to Bassics article.