PCB Who?

Hi all,
         I was scouting around the other day trying to find a big
transformer for a future tesla coil and it turns out that i can probably
get 2 pole pigs for free :). Problem is that they are rather old and the
person I was talking to siad that they most likely contained PCB's. Ive
heard that PCB's are bad juju but I'm not really sure what they are. Is
there anyone who can explain just what a PCB is and what it could do to

                                                Thanx Heaps