With the advent of Web 2.0 and AJAX the way in which users interact with web pages has changed.
Increasingly in Web 2.0 application users can interact with much more than the common elements of HTML designed for interaction, so rather than clicking on links users can interact with divs and spans too. This has come about simply because HTML, which was primarily designed as a mark-up language for documents, isn't ideally suited to providing rich internet applications.
The W3C's standards draft for Accessible Rich Internet Applications (ARIA) addresses these limitations by providing new attributes to convey the role, meaning and importance of elements on a page as well the relationships between elements. These machine readable additions allow the interactive parts of a web page to be seamlessly mapped to the users Operating Systems Accessibility API in the same way that desktop applications do so.
For example web designers might prefer to use a div layer for a dialog box rather than the default browsers dialog box, simply because it provides more design flexibility. ARIA allows the designer to attach a role of "dialog" to the div this therefore indicates that this item should be treated as a dialog box. Other allowed role attributes are: progressbar, slider, button, tree, textfield, checkbox, alert and dialog. There are many examples on the Mozilla site.