Google, Bing and Yahoo jointly announced a new initiative today at Schema.org to create and support a common vocabulary for structured data markup on web pages in Microdata.
For many years search engines have worked independently of one another to establish some standards in structured markup. Essentially, through Schema.org, the search engines have come together to provide support for a common set of schemas - again, that would be microdata.
In the past, there were different standards for structured data markup including microdata, microformats, and RDFa, causing confusion for webmasters which may have ultimately been the cause of the slow adoption.