The meta description tag of a web page is often used by search engines as the text snippet on the search engine results pages, like this:
Tips for meta description:
As mentioned below, Yahoo may pay more attention to meta description than the other engines.
Do search engines read meta keywords? Yahoo does. Apparently Yahoo doesn't use them for ranking purposes, but only to determine whether a page should show up for a certain query.
SEO-lab.com ran a test that seems to indicate that Yahoo does pay attention to meta keywords and meta description.
I don't believe that meta keywords are given much weight. I think that a different combination of meta keywords should be included on each page, but no more than 10 or 15 words. I don't think that meta keywords are worth spending much time on. The important meta tag is the meta description because it is a chance to put your custom text on the search engine results pages.
The Yahoo meta keywords experiment was replicated here. Today, the only result for the nonsense word in the meta keywords is this page: