What is Microdata?

Microdata is a WHATWG HTML specification used to nest metadata within existing content on web pages.Search engines, web crawlers, and browsers can extract and process Microdata from a web page and use it to provide a richer browsing experience for users. Search engines benefit greatly from direct access to this structured data because it allows search engines to understand the information on web pages and provide more relevant results to users.

Microdata Vocabularies:

Microdata vocabularies provide the semantics, or meaning of an Item. Web developers can design a custom vocabulary or use vocabularies available on the web. A collection of commonly used markup vocabularies are provided by Schema.org schemas which include: Person, Event, Organization, Product, Review, Review-aggregate, Breadcrumb, Offer, Offer-aggregate. Major search engine operators like Google, Microsoft and Yahoo! rely on this markup to improve search results.
Microdata global attributes:

itemscope – Creates the Item and indicates that descendants of this element contain information about it.
itemtype – A valid URL of a vocabulary that describes the item and its properties context.
itemid – Indicates a unique identifier of the item.
itemprop – Indicates that its containing tag holds the value of the specified item property. The properties name and value context are described by the items vocabulary. Properties values usually consist of string values, but can also use URLs using the a element and its href attribute, the img element and its src attribute, or other elements that link to or embed external resources.
itemref – Properties that are not descendants of the element with the itemscope attribute can be associated with the item using this attribute. Provides a list of element ids (not itemids) with additional properties elsewhere in the document.

*** As of December 2013, notable browsers that added support for the Microdata DOM API.