Through the use of technology, we have developed methods to update information in the browser window dynamically without manually refreshing the page from the server. For a person gifted with sight, this a boon, retrieving information quicker and in a more useful way. But what happens when a person cannot see? How will they know information on the page has been updated in any way without refreshing the page, redisplaying its contents, and having the assistive technology read it to them again in its entirety?
Accessible Rich Internet Applications (WAI-ARIA) is an emerging technical specification that, like many of HTML5's new semantic tags, forces us to really think about our content and how we want to present it to our audiences. We can use WAI-ARIA to define roles, properties, and states to help us define what our elements are supposed to do.
The WAI-ARIA Overview at http://w3.org/WAI/intro/aria, based on Marcotte's Responsive Web Design approach.