In the video on this page, Can I tell Google not to use the posting date in my snippet? –  you will see Matt Cutts’ presentation about Google Search features. He discusses Google Search snippets and their design intent.

PageRank is a set of rules and they are used by the prominent Google Search web tool to rank website search result importance. The Google Search engine does this by using spiders. Spiders are automated scripts that crawl the web and rank every website according to proprietary rules. These rules are determined by many proprietary algorithms. Some of these algorithms may include the use of hyphens and underscores in key website locations.  Although these rules have much complexity, they are designed to aid the user in their search experience.

The intent of these complex search rules is to make search results informative and relevant to the user. For example, if we use Google to search for the term “laptop” we should first see websites related to “laptop computers” and not the definition for a “lap” in our results. Also, if additional site information is available the user may be able to mouse over the site link and Google Search may display additional site information in a snippet. Some features of this type can be customized, so expert search engine user interface advice is helpful.

Google software engineers are experts on Google Search engine rules. One such expert is Matt Cutts. You can use his information to optimize your website for high Google Search rankings. Matt has taken the time to explain Google Search snippets and how they are intended to be used. You can see this information in the video, Can I tell Google not to use the posting date in my snippet?