Hittail is a free analytics tool that tells you what keywords your visitors are using to find your site. It's actually far more intuitive than that simple statement. It is centered on longtail keywords.
It gives you a list of keywords that your web site ranks for and allows you know if you are in the top ten for those terms. It tells you how many times somebody has clicked on that particular term to visit your site.
It gives you the exact domain name that your visitors arrived from. This is helpful because you are able to visit the URL to see where your link is.
It gives you suggestions of what you should optimize your site for. Longer, more focused search terms generally have a higher rate of purchase.
Nobody has a direct understanding of why search engines rank websites for certain terms. Research has been done and there are assumptions, but the proof is in
your personal experience with search engines. I have friends that have ranked for words that are never even mentioned on their pages. If you search the term “click
here”, it leads to adobe’s website. This is because people use that term in the anchor text links pointing to their website.