lorax - 8:37 pm on Oct 22, 2012 (gmt 0)
Keywords have SEO value and are typically the META Keywords hidden from the user but readable by them if they view source for any page. These are read by the search engines to help rank the page they are used on.
Tags are more like onsite filters. You use 3-4 tags per post to help describe the key subjects/topics within the post. They could be thought of as keywords but the intent here is to make these generic enough they can be used again and again.
So if I had a site about sneakers a keyword for a page might be the product name: New Balance 1260. The tags might be "running shoes, stability, over $100"
The point of the tags is to help your readers with locating similar content.