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
You can add this meta tag to stop all robots from using DMOZ and Yahoo directories descriptions as your
<meta name="robots" content="noodp,noydir" />
More info here