Pickups can be either - "Active or Passive"
Single-Coil Pickups have one coil wrapped around the pickup's magnet. ==> Pro: They are bright and clear sounding ==> Con: They can pick up external noise and give off a humming sound.
Split coil pickups are wired to hum cancel while each string is sensed by only a single coil.
Double-Coil (5-string pickups):
Double-Coil Pickups: Just as two single coil pickups can be placed side-by-side to cancel hum, a double coil pickup can be created to cancell the hum within one pickup housing. These pickups are often called humbuckers, or humbucking pickups, for their hum-reducing qualities. ==> Pro: Usually have more output than single-coil pickups ==> Con: Tend to roll off some of the tonal highs when cancelling the hum.
Soapbar Pickups, refers to the shape of the pickup's housing being compared to bars of soap, black in color, designed for 5 & 6 string bass guitars.
MM-Style pickups, refer to Music Man pickups created by and used on Music Man bass guitars. Pickup manufacturers have designed MM Style pickups for any bass that the housing cavity will support.