The metrics sent from the users browser include:

• The users IP
• Page URL and Title of the page they visited
• The browser type they used
• The Referer (from which site they link to yours in case it was not a 'direct hit')
• The Search Keywords (in the case where they hit your site via a search engine)
• The number of elements on the page
• ...and most importantly the load time of the page

This informtion is processed further at the WebTuna data center so that the IP address can be mapped to location and the company or ISP who owns the IP address.