Custom Search themes make it easy for you to customize the look and feel of your search results pages. And if you want to take the customization gig further, you can also customize the result snippet—a small sample of content that gives search users an idea of what's in the webpage—by using structured data.
When you are reading a webpage that reviews a film, you can figure out what the title is, what reviewers thought of the film, and how they rated it. You can even search for stores with the best prices for the DVD. Structured data can convey the meaning of such key information to computers.
Structured data formats—such as microformats, RDFa, and PageMaps—are semantic markup that you add to your HTML page. Structured data make web content more meaningful to machines. These attributes do not change the formatting of your website, they just make the text enclosed within the XHTML tags "understandable" by computers and influence what shows up in the result snippets.
When you tag your webpages with structured data, Custom Search indexes them and sends the metadata back in the XML results for your page. You can then take this XML feed and transform it into HTML results that showcase key information—such as image thumbnails, summaries, dates, authorship, ratings, and prices. Having the most relevant information in your search results makes the webpages in your site more compelling to your users.
You can, for example, create the following kind of rich snippets:
You can even add thumbnails and actions that let your users download files or make purchases.
To learn more, read the Custom Search Developer's Guide.