What is the difference between digital pen and smart quill?

A digital pen is little more than a stylus with more advanced features. Like a stylus, it is meant to be used with a tablet of some sort, but may also have multiple buttons and a blunt end which can be used as a digital eraser.

The smart quill is a proposed device which can be used as a normal pen, even writing on paper, but can also record the movments and thus writing digitally to be retrieved later on a computer system.

It should be noted that as of 2009, the smart quill has not progressed beyond the conceptual phase and is not commercially available whereas digital pens have been in production by companies like Wacom for years.