A page's description meta tag gives Google and other search engines a summary of what the page is about.
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. This gives
the user clues about whether the content on the page matches with what he or she is looking for.
Avoid
writing a description meta tag that has no relation to the content on the page
using generic descriptions like "This is a web page" or "Page about baseball cards"
filling the description with only keywords
copying and pasting the entire content of the document into the description meta tag
using a single description meta tag across all of your site's pages or a large group of pages
