To add to rockonn91's post, a humbucker is basically 2 single coils put together. When 2 single coils are put together like that, they cancel out the hum that is associated with single coils....That's how they they got the name humbuckers.......they buck the hum.

Single coils pickups are much thinner sounding while humbuckers are fatter.

Those are the 2 most common types of pickups...other types include Soapbar pickups (looks kind of like a humbucker) and lipstick pickups (looks like a metal bar).

Then there are active and passive pickups. These can be single coils, humbuckers, etc......the difference is that active pickups require a 9 volt battery and are much more sensitive than passive pickups. Most guitars have passive pickups. They don't require the battery......

EMG has the best active pickups in most people's opinion. A lot of the metal players use these such as Zakk Wylde, James Hetfield, Kirk Hammett.........