According to Google "Website Optimizer is a tool that can help you improve the effectiveness of your website in getting a return on your investment. By allowing you to test different versions of your site content, you can determine what will best attract users and lead them to convert on your site.

Website Optimizer can help to improve your conversion rate and increase your ROI by enabling you to figure out what users on your site respond to best.

Website Optimizer works by testing different versions of the content on your live site to see what will lead to the highest conversion rates. You provide us with the original content of your site and the different variations of that you'd like to try out. We run an experiment on a portion of your site traffic using those variations, to see the effect of each variation on your conversion rate. When we've collected sufficient data, we'll provide you with a full report of our findings that can help you determine what changes to make to your site"