pickups derive their name from the fact that they eliminate or "buck" the
60 cycle hum that plagues standard single-coil pickups. They do this
by wiring two coils such that the hum of one coil is canceled by a "mirror
image" of the hum from the other coil. These pickups are also, with
few exceptions, wired in series (whereas single coil pickups are usually
combined in parallel), such that the sound produced tends to be warm and
thick. However, a tremendous amount of variety exists in the market,
so almost any color of pickup is available if one is willing to look.
Another distinction between humbuckers and single coils is that one or
both coils of a humbucker typically have adjustable pole pieces.
These can be adjusted with a screwdriver (or Allen wrench, as is the case
with DiMarzios) to conform to the string height. Of course, there
are dozens of variations in appearance, including the minimalist look of
EMG pickups or the aforementioned "stacked" humbuckers that look (and often
sound) in every other way like single coils models. Variety is the
spice of life!