Making your website accessible is essential so that everyone can use it (and is required by law in many cases). However, some functionality currently used in Web sites is not available to some users with disabilities, especially people who rely on screen readers and people who cannot use a mouse. W3C's WAI-ARIA addresses these accessibility challenges, by providing a framework for adding attributes to identify features for user interaction, how they relate to each other, and their current state.
WAI-ARIA defines new ways for functionality to be provided to assistive technology. It includes technologies to map controls, Ajax live regions, and events to accessibility application programming interfaces (APIs), including custom controls used for rich Internet applications. WAI-ARIA techniques apply to widgets such as buttons, drop-down lists, calendar functions, tree controls (for example, expandable menus), and others.

For more info, see: WAI-ARIA Overview