ARIA is Accessible Rich Internet Applications, a set of standards under development by the W3C. It’s part of the WAI (Web Accessibility Initiative).
ARIA provides ways to describe roles supported by content. Roles include such things as “menu,” “treeitem,” “slider,” and “progressmeter,” Roles can also describe structure, with roles like “article,” “heading,” “complementary,” and “rowheader.”
ARIA provides properties that describe state. State properties can include things like “checked” for a check box, or “haspopup” for a menu. Properties can also describe parts of a page that may be updated—for example, stock quotes—or properties for drag and drop resources.
Providing information about roles and properties makes the page more accessible. The WAI-ARIA Overview states,
Roles and properties are added as properties to HTML elements. Examples:
<div role="button" aria-haspopup="true">