piezo pickups

hey guys, I have a bit of a n00b question to post. I've seen in adds that certain guitars feature a 'piezo bridge pickup.' What's so special about this? What kind of tone does it give? I've gathered that it's got something to do with an acoustic sound.

Thanks for being patient.

A piezo-pickup is a contact-type pickup, as opposed to the more frequently used magnetic pickup on electric guitars.
On electric guitars, this kind of pickup is supposed to give a more "acoustic"-sounding sound.