Just referring to the google site, got description to Google Website Optimizer as,
Use Website Optimizer to test different website content. Graphical reports show which content engages your audience and results in the highest conversions. Run that content to improve your users' experience and convert visitors to customers. Complete the steps below and sign-up to participate in our beta test. We can only invite a small number of advertisers at this time, but hope to open the tool to all Advertisers over the coming months.
Basically its a tool that enables you to create "experiments" which are nothing but different content/images for different section of your page. The difference is, you define it inside the tool by google, and while experimenting different things, you dont have to change anything in your website code, Google tool does everything for you.. Google also collects the data for each experiment and then gives you reports as to which experiment was the most successful.
Take course on Google Website Optimizer over here
Find more about Google Website Optimizer here