The defining characteristic of a password field is that it abstracts text as dots. While the intention of this behavior is understandable (it makes users feel secure and protects from prying eyes), the unintended effect is that it creates a usability problem. Users can't tell if they've entered a password incorrectly until after the site's validation informs them. It's like typing with your eyes closed.
The author describes and provides demonstrations to three alternate methods to approaching password confirmation.