I keep having to dig this info out, so I’m putting it here for posterity. This is to help us web devs, and web designers, content creators and editors generate great SEO-centric meta data for websites.
Less than 70 characters. This is the limit Google displays in search results. This should be relevant to the content and generally not start with the name of the company or website (there are some exceptions to this rule of course).
Less than 160 characters. They can be any length but search engines generally truncate snippets when descriptions are longer than this. The focus of this should be a compelling reason to click on the link, rather than keyword stuffing.
Largely unused now. They are not used for searching or ranking, but competitors can use them to see what keywords you are targetting!
The other items are very focused on getting clicks from a search engine result, but it’s good content that will get you high up the rankings. Make it relevant, short, snappy, and make good use of headings. We won’t go into semantic markup here, this is a quick reference!
Want some further reading? A good place to start is [SEOmoz](http://www.seomoz.org/learn-seo/)
(of no affiliation to me, it’s just a good resource)