I have been around the block a few times and I have NO IDEA what a dental dam is...and I do not think I want to know.
It's the equivalent of a surgical drape for oral surgery, particularly root canals, used to maintain a sterile field in the operating area. A thin piece of rubber; imagine a rectangular piece cut from a child's balloon, but slightly tougher.
If you're ever at the dentist and hear those dreaded words, "You need a root canal", you'll find out for yourself. (Actually, I was surprised at how painless mine was, until I got the bill. ;-))