This is probably the most important of these meta tags as this is where you will be placing keywords that you would like search engines to use when users punch in these words when looking for a product or service.
For example, you provide brown paper bags at wholesale prices. A user is looking for "brown paper bags". If you place these keywords at the beginning of your keywords, at the beginning of your description and the title of the page and if you also place emphasis on these words in your content there is a stronger likelihood that your page will be presented to the user in question. However, meta tags and content alone do not guarantee this. All elements of SEO have to work together seamlessly in order to make your site more effective.
Now you want to tell the search engines what keywords you would like to be found under. Current thinking is that when it comes to meta tags, less is more. Overloading your keyword meta tag with a diverse collection of keywords only waters down their impact on your rankings. Try using only 2 to 5 top keywords / phrases on each page, but use different ones on different pages. Use phrases rather then separate words. The Keywords Meta Tag isn't any longer as important as it was in the past. Even Altavista doesn't read this particular meta tag any more. Repeat a few keyword phrases you have used in your title and description.