Google's Snippet

Google uses your meta description if its relevant, however, sometimes if you are listed in DMOZ it will take information about your site from there, to use as your snippet. You can force google not to use the DMOZ snippet, by adding

<meta name="robots" content="noodp" />.

Yahoo can make the snippet from the yahoo directory, you can force the search engine to ignore the ODP information by adding... 

<meta name="robots" content="noydir" />

To Block Yahoo Directory and DMOZ from being used by all search engines.

You can add this meta tag to stop all robots from using DMOZ and Yahoo directories descriptions as your snippet.

More info here

http://www.seoworkers.com/seo-articles-tutorials/using-noodp-and-noydir.html