Your meta description tag is a valuable part of optimizing your site for Google that is often overlooked. A description tag is basically a snippet of information on each page of your site that shows up in the search results below the heading of each search result (see picture below). The description tag does not show up on the actual page of your site in its readable text but rather is something that you enter for each page on the site. At this time the length of the description tag shown by Google is 150 characters.

Best practices for description tags:

1. Unique description tag for each page on your site.

2. Use each page as a chance to rank on Google for a phrase.

3. Use your target keyword(s) phrase in your description tags, preferably toward the beginning.

4. Use your targeted keyword phrase once, perhaps fitting in secondary phrases if possible.

5. Use this tag to continue to interest the reader and give them extra reasons for clicking on your listing. Think of this tag as valuable "real estate"- like a billboard that you can use to your advantage to get more clicks than you're competitor even if they are above you in the search results.

6. Keep the description under the above amounts of characters to make your message complete and not trail off...

7. Know that Google will not always use your description tag in the search results- see the following article:
http://www.wordstream.com/blog/ws/2009/11/19/meta-description-mutiny

Additional Links:

Google's page on what they look for in the meta description tag and how best to optimize it:

http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=35264