Keywords META tag:
This tag contains keywords relevant to a page. In the past search engines referenced this to determine how to rank a particular page. However many people started to abuse this and stuffed the tag with keywords that were not relevant to the content of the page. This confused search engines and subsequently returned weaker search results. To combat this less and less emphasis has been placed on the Keywords META tag. However this does not mean that it is entirely useless. Here are some reasons to still include it:
* Some search engines still use it
Although most search engines do not use the tag there are undoubtedly still some that do. Do not attempt to cheat the search engines by repeating keywords as this is considered spamming and they may look unfavorably on this.
* Account for misspellings of key phrases
People can misspell certain key phrases that you may be optimizing for so it is common to place these misspellings into the Keywords META tag. This allows you to rank in search engine results for misspellings without having to represent them on the actual content of your page.
Description META tags:
In the past this tag was used to specify the text that would appear in search engine results alongside the link to your site. Keywords could be placed into this text to influence ranking. However it was abused similar to the Keywords META tag and has befallen the same fate, meaning it is not as useful as it once was. Some search engines do not display it in their results (most notably Google, which retrieves the text from content on your page) and do not use it to rank a site.
Descriptions placed into this tag should, strangely enough, be very descriptive of the page or site they reside in. Include a few relevant keywords/key phrases that you are optimizing for in the description and try to limit its length to 25-30 words. Also try to use no more than two sentences.
Keywords META tag : 15-20 keywords, include misspellings of keywords/key phrases
Description META tags : Include keywords/key phrases, 25-30 words, and no more than two sentences.