SeoPapers - article The basics on Meta tags.

The Meta tag is not nearly as popular with search engines as it has been, however Inktomi, who will soon be the exclusive search engine for Yahoo, still uses Meta Descriptions. Because of this, it's still worthwhile to give proper attention to the META tags.

Meta tags are most often found inbetween the <head> and </head> tags. The Meta tags allow you to tell the search engine spiders a bit more on how you would like them to display your site. When a user enters a search string and the Search Engine Results Page (SERP) returns your site, if the Meta Description is declared, it will use this on it's site.

The two most important Meta tags is the Description tag, and the Keywords tag.

The meta description should include a factual description of your site. You should, as a practice, have your main keywords (up to 3 words) in the first 5 words of the meta description. This practice will help your keyword proximity and ranking. Although you want to use a lot of keywords in your description, it is equally as important to ensure you do not repeat a lot of words, you should not use the same word more than twice in the description to prevent the spiders from labeling the description as spam.

The second meta tag is the keywords tag. The keywords tag was used to further assist the search engine spiders in indexing your site. It is good to remember that having keywords in the meta tag that are not in the content of your page will be completely useless. You should only use keywords in the meta tag that are also in the body of your website. Do not try to stuff repetitive keywords in the tag, as spiders will detect this and not index your site.

Other non-SEO Meta Tags:

This tag is used to instruct the browser on how often to refresh the page. The main variable 'content' is the frequency in seconds you want the page to be refreshed.

This tag is used to instruct the browsers and spiders on when this page is out of date. This tag is completely ignored by spiders however. The variable 'Content' is the date of which the page is out of date.