Microdata is a method of marking up elements with additional machine-readable data, so that crawlers, search engines, or browsers can extract information from the page. Unlike WAI-ARIA, it’s actually part of HTML5.
With Microdata, page authors can add specific labels to HTML elements to annotate them so that they are able to be read by machines or bots. This is done by means of a customized vocabulary.
It’s similar to RDFa (a W3C standard) and microformats (a popular set of conventions), and is already indexed by the Google search engine if used in markup.