Site descriptions in Google results are actually quoted from the web page in question. Google automatically generates different descriptions based on the search terms used to find the site (these “snippets” display the search term(s) in the context of the page on which they appear).
For example, if there is a pet site that deals with cats and dogs, and someone enters a search for the word ‘dog,’ the site description on Google will only talk about ‘dogs.’ If a person searches Google for ‘cats’ and the same site is delivered as a result, the description will be different – it will contain references to the word ‘cat’ as it appears on the website.
Google does not display a standard description. We look for the search terms specified (and in some cases, variations of those terms) and show snippets of where those terms appear. This is a completely automated process and editing is not an option. If you alter the relevant text on the page itself, Google will pick up those changes during our next crawl in a few weeks.
Excerpt taken from Google Webmaster Info