I've heard my girlfriends talking about dental dams -- what are they exactly?

Dental Dams: These are used by dentists during procedures to create a sealed off environment in the mouth. They are rubber or silicone thin sheets that sexual partners can use to create a barrier between genitals and mouth, fingers, tongue, etc. They can aid in the prevention of std's, when used consistently and properly.

A dental barrier: Dental dams are used as barriers during a dental procedure. A tooth or several teeth are isolated with the dam so when a drill is used, the "stuff" such as a mercury filling doesn't enter your mouth , and when the tooth or teeth are restored, they will not be contaminated by your saliva. Also use for root canals for the same reasons.

Rubber sheets: Rubber dams are used to isolate teeth while working on them. You use a clamp around a tooth to achor them.

A barrier: Typically they are latex or latex free squares that are used for isolation of a tooth during root canal treatment or restoration. Another common use is to help prevent stds by using as a barrier during cunnilingus. Depending on what you're needing it for, either have fun, or good luck at the dentist.