Mel Pedley of Accessites.org has posted an excellent article on the difference between web accessibility and web usability.

Whilst there is no doubt that the two topics do overlap to a significant degree, it is important to differentiate between them. Unlike web accessibility which impacts directly upon disabled users, web usability affects all users, and can be defined as a measure of how easy it is for a generic site visitor to carry out a task such as finding a given piece of information or buying a certain product.

Web Usability goes on to explore key components which make a site usable.