Usability is of course a fundamental part of a user experience.
I see UX as a result of any combination of different fields (interaction design, user interface design, information architecture etc.) that together shape the eventual experience a person has, when using a product. Therefore UXD is an all encompassing design process of the above mentioned fields.
In other words, user experience design is more than just interface design or usability design. It’s designing the overall experience a person faces when using a product (whether good or bad).