Summaries can be defined for each page:
A page's description meta tag gives Google and other search engines a summary of what the page is about (1). Whereas a
page's title may be a few words or a phrase, a page's description meta tag might be a sentence or two or a short paragraph. Google
Webmaster Tools provides a handy content analysis section that'll tell you about any description meta tags that are either too short, long, or
duplicated too many times (the same information is also shown for <title> tags). Like the <title> tag, the description meta tag is placed
within the <head> tag of your HTML document.
What are the merits of description meta tags : Description meta tags are important because Google might use
them as snippets for your pages. Note that we say "might" because Google may choose to use a relevant section of your page's visible
text if it does a good job of matching up with a user's query. Alternatively, Google might use your site's description in the Open
Directory Project if your site is listed there (learn how to prevent search engines from displaying ODP data). Adding description meta
tags to each of your pages is always a good practice in case Google cannot find a good selection of text to use in the snippet. The
Webmaster Central Blog has an informative post on improving snippets with better description meta tags.
Words in the snippet are bolded when they appear in the user's query
