Gez Lemon from TPGi has written an article for Dev.Opera, which provides an Introduction to WAI ARIA.
WAI–ARIA is a specification that provides a means of describing roles, states, and properties for custom widgets so that they are recognisable and usable by assistive technology users. WAI-ARIA also provides a mechanism to ensure that users of assistive technologies are aware of updates in the application. This article provides an overview of what web applications should do, and how WAI-ARIA helps.