The rich snippet markup tool added by Google was well received when it was launched back in December 2012, but it was limited to event-type data.
This month Google added six new data types including those for articles, movies, TV episodes, local business, software applications and, finally, products. Strangely, the link to the product data type listed in Google Webmaster Help has been broken since the addition, leading many to believe that Google is working on something unique for Internet retailers which it will release in the near future (more on that as it develops).
Google's Data Highlighter tool is essentially a WYSIWIG editor for structured markup, enabling authorized site owners to add rich snippets to Web pages - which previously had to be hand coded and added to pages on an individual basis.
To tell Data Highlighter which of the pages on a site contain data (data that can be used for rich snippets which appear on the search results), webmasters must create one or more page sets - a collection of pages on a site that display data consistently (possibly generated from the same template) and are organized so that URLs follow a simple pattern. The Data Highlighter will only teach Google about pages that are in a page set.
Also keep in mind that the Data Highlighter can only access pages that have been crawled by Google recently. If Google has not or can not crawl pages, such as those behind sign-in forms, the Data Highlighter can't be used for those pages.
The data highlighter tool is available within Google Webmaster tools.