Duplicate makes a copy of an item, while instancing is kind of like a ghost of that item. The copy can be posed and changed independently from the original, while the instance is pretty much an image of the original, so anything done to the original will also be done to the instance of it. An instance doesn't take up any more resources than its parent (at least not to a noticeable degree) while a copy does because it's another physical object in the world space.

Hope that helps to clarify.

**Some programs do allow changes to be made to instances while the original remains unchanged.

It does help some, Shane. I know several people have called for instancing in Poser, but I'm kind of torn. There are times when instancing sounds like a good feature to have, like creating a crowd from a distance.