Humbuckers usually have a brighter, "fatter" sound.

Single coils are more "natural" sounding, and IMO more mellow.
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The humbuckers are going to have a thicker, bassier sound usually, and could do metal and rock a bit better too. While the single coils will do blues, jazz, country, and classical quite well. They can however, be used for rock, and sometimes metal. Examples being Iron Maiden, and Deep Purple.

That's just a generalization for you though to sum them up, as all pickups are quite different.
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Last edited by zappp : Today at 4:20 PM. Reason: Suck on my balls, UG