They're balls, made of metal or plastic, which are attached by a string. They can be inserted into the vagina or anus, and left in to provide pleasure, or removed at varying speeds to have the same effect. Some of them are hollow with a weight inside them which moves around, which some people enjoy.
Wikipedia has a good entry describing them. I didn't know Ben Wa was a brand name until I read it.
Unfortunately, I have no experience with them. They seem pretty interesting to me.
-Tamerlain