There is a lot of mystery surrounding meta tags and their affect on SEO. I guess it simply boils down to the fact that there has always been a lot of mystery around SEO in general, and meta tags were originally what 'worked' to get sites and pages ranked high in search engines.
In the beginning, search engines like Metacrawler and Search.com simply required that you place the proper meta keywords and description on your pages, and they'd reward you with high rankings. Of course, spammers quickly exploited this to get all of their pages ranked for whichever terms they wished - relevant or not. Then a couple guys built a search engine called Google, determining true relevancy by looking at all factors of on-page optimization (not just meta tags), as well as the anchor text in incoming links. Later on they dumped meta tags as ranking factors, and other search engines followed suit. Yes, it's an over-simplification of the history, but it gets me to where we are now.
At this point in search engine algorithm evolution, meta tags (keywords and description) are virtually useless when it comes to affecting ranking. Google doesn't even bother with the meta keywords tag anymore, nor does MSN/Live Search. Yahoo! apparently does still give it some weight, but it's incredibly minute.
Meta Keywords
In the SEO community, it's pretty much accepted that that the only use left for the meta keywords tag is to target misspellings. For example, say you've optimized your site for the keyword "socks". Perhaps you'd like to also rank high when people accidentally type in "sokcs", but you don't want to misspell it on your page and look bad.
You could include "sokcs" in your meta keywords tag, and you may be able to obtain some ranking based on that. But honestly, now that search engines endeavor to correct mistakes (e.g. "Did you mean: socks"), most people would be more inclined to click on the correct search term than on a result of their mistaken query. Therefore, you'll probably get very few clicks from most misspellings. Therefore, the time spent on your meta keywords may very well be wasted. Therefore, you can safely forget about the meta keywords tag!
Meta Descriptions
The meta description tag does still serve a very valuable purpose. You've probably noticed that some search results display an excerpt of content somewhere on the page, which relates to your query. As this content is excerpted, and sometimes pieced together, it doesn't always make sense. For example, here's the excerpt of justin-cook.com, when someone searches for "justin cook":
Site by Justin Cook | RSS | Disclosure policy Comprehensive resources on PHP based cheap web hosting at very reasonable price. photo of Justin Cook.
This isn't the smoothest looking content, it doesn't even seem to make that much sense. That's because the search engine has excerpted what it thinks to be the most relevant portion of the page, and thrown it into the description. I probably wouldn't click on that result if I was looking for myself!
That's really where the meta description tag comes into play. If a meta description tag is present on a page, a search engine will use that instead of piecing together its own description. This means that you have the power to create an attractive, keyword-focused meta description on each page of your site. No, it won't change your ranking. But yes, it will increase your traffic. Think about it: if two sites are ranked at #1, one having a nonsensical excerpted description and the other a clean and succinct description that the searcher identifies with, you can be sure that the searcher will click on the description he/she understands and identifies with better.
How to write meta descriptions
When writing your meta descriptions, keep them between 150-250 characters in length. Anymore will be cut off, anything less may not be indexed. Don't make it spammy, make it easily readable and attractive, something that you yourself would be enticed to click on.
One big word of caution - don't use identical meta descriptions across your entire site! I see it happen quite regularly, and it's a very easy way to get the pages of your site dumped into the supplemental index! It also turns people off when they are searching for a specific product/service, and all they see is a generic description of your company, as that's what all of your meta descriptions contain.
Conclusion
Hopefully you now understand your meta tags a little better. You now know that you can scrap your meta keywords if you want. You are equipped with the knowledge to craft your own unique, enticing meta descriptions. So go ahead, get to work!
Oh, and one more thing. If an SEO company is pitching to you that they will do your meta tags for you to boost rankings, show them the door!
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