I have heard of a piezo pickup used in some guitars, like John Petrucci's signature Music Man.
What is a piezo pickup and how does it differ to a common pickup?
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A piezo pickup is a type of microphone. It picks up the physical vibration of the the instrument and converts it to a voltage. It is usually used on acoustic guitars/instruments.
A "common" pickup - generates a signal by disturbing a magnetic field generated, you must have metal strings for this to work.
There's a very good page covering pickups on WikiP : Wikipedia