"Contextual information allows search engines to understand the meaning of the information presented on your website, which allows it to better answer more verbose 'natural language' queries where an understanding of the meaning helps to interpret the most relevant content to be displayed.
"Microdata can be used to explain just about anything you would ever want to explain, and there are more 'schemas' being added on a regular basis." Source: Joomla! Documentation.
Schema.org is the current favourite.