active uses a low output pickup, and a preamp that boosts the signal using a battery. most of the time, these have low output impedance.

passives are just normal pickups, with no batteries, alnico or ceramic bar magnets used.
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Actives have a preamplifier circuit inside the guitar that requires power from a 9v battery(ies) or external power source, typically.

The output from the guitar is typically a lot stronger than a passive pickup guitar... which is all straight signal.

EDIT: You beat me to it..
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