What is a piezo pickup and how does it differ to a common pickup?

Question

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?

Answer 1

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