There was a time when keywords were very important in web sites promotion and ranking. The way search engines spiders considered the content of a page for ranking and appropriate search matching, was strictly related to keywords. But then, many webmasters thought that filling up long lists of not so much related keywords to their pages, helped them to get a higher ranking. When I say "not so much related", I mean that sometimes a good and sexy word could really draw traffic. Webmasters used to look for popular keywords and add them to their pages, regardless that they were consistent or not with the content of the page itself. And so, starting from 1999, search engines have gradually stopped looking for them and stopped considering them. At least most of them (it actually seems that Yahoo is still using them in conjunction with the real page content).
Unlike keywords, the description attribute seems to be taken in account and seems to be supported by all major search engines. According to w3schools "A lot of webmasters have used <meta> tags for spamming, like repeating keywords (or using wrong keywords) for higher ranking. Therefore, most search engines have stopped using <meta> tags to index/rank pages" (quoted from the HTML Meta Element page).
However in my experience, it is quite important to use an appropriate description meta tag to improve ranking. Just to be clear "appropriate" means that if you use keywords in the description attribute, those keywords must be on the page as well - at least this is quite sure for Google. So Google seems to ignore meta description when its content is not present on the page as well.
There's actually a big debate on the matter, and people are trying to validate or, sometimes, to disprove those findings. It's quite clear that Google itself would not disclose if keywords or description attributes are actually used by spiders. That is because, considering the possible misuse of keywords, it wouldn't be wise to admit they use them for indexing and ranking.
According to SEO experts, and I actually agree with them, as long as you use keywords and description attributes appropriately, they might not help you very much, but they absolutely won't damage your web page's ranking and indexing. Sometimes I have read that, unless you know how to create it, the meta attributes could cause damages to page ranking, and sometimes it seems that in those cases it is better not to insert them in the head of your document.
My opinion is that:
- if keywords and description attributes are used appropriately, they actually help a lot;
- if keywords list contains words that are actually on the page (I would say on the first part of the page), they are taken in consideration and help indexing;
- if the description's content is consistent with the page content, it will help people finding your page in the SERP (Search Engine Results Pages).
I suggest to:
- describe the page content accurately - this will help people to click your page in the SERP;
- include all the relevant information (e.g. for a book catalogue, include the book title, the author's name, the year of publishing and so on);
- do not repeat words, do not insert URLs or the page title again (it is already there in the title meta tag!);
- try to be precise, as if you were actually describing something to someone who knows nothing about it.
If you need more info on the matter, Google has got a good document on search engine optimization.
This is what I think about keywords and description attributes. I need to remind you that search engines policies are unfortunately changing very quickly and often. The information provided above are probably relevant today, but they might be not relevant anymore tomorrow. Remember that.