What is "rich-snippets"

Rich Snippets are information inserted into the HTML structure. The purpose of this information is to organize and classify content on the web using Structured Data. With them we can give some buzz to google at the time of indexing the site, telling him what kind of content the pages bring. Rich snippets can be created through three types of markup: microdata (recommended), microformats and Rdfa.

Official link with instructions for use: Official instructions

Link to test tool provided by google: Online test