To add.. some people add SOOOOO MANY keywords to the meta tags that some search engines are starting to not factor them in as heavily.... some designers will put competitors name brands into their meta tags so their page shows up when you run a search on the leading name brand.
Search results used to be ranked depending on your search compared to the frequency of your search within your meta tags...
Therefore, if I used the word "Viagra" 50 times in my meta tags and you used the word "viagra" 40 times in your meta tags, I would get a higher rating on the search results list.
However, more and more, search engines are actually basing their results on the frequency of your search to the content on the actual home page (in the <body> section). For instance if My page had the word "Datazing" listed 1 time in my meta tags but listed it 10 times on the home page, I would get a higher rating than someone that had "Datazing" 100 times in ther meta tags but only once on their home page.
In Some search engines totally ignore sites that list the same word a billion times over in the meta tags section.
In other words, keep your meta tags short... about 25 words/phrases or less and go heavy on the homepage.