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A snippet is a text excerpt from the returned result page that has all query terms bolded. The excerpt allows users to see the context in which search terms appear on a web page, before clicking on the result. Users are more likely to click on a search result if it has a corresponding snippet.
If you wish to prevent Google from displaying snippets for your pages, use the following tag:
<META NAME="GOOGLEBOT" CONTENT="NOSNIPPET">
Note: removing snippets also removes cached pages.
Of course, I could be wrong. I haven't paid much attention to it in a very long time. If anyone is using the no snippet tag and getting their meta descriptions displayed instead, I'd love to hear about it.