Google Dashboard is a new service that shows a summary of the data stored with a Google account. You'll soon find a link to Google Dashboard in the "personal settings" of the "my account" page.
The dashboard lists all the information associated with the Google services you use: your name, your email address, the number of contacts, the number of conversations in your Gmail inbox, your Google profile, the most recent entries from the web history etc. It's a long answer to the question: "What does Google know about me?".
More information about the new service in a YouTube video that was supposed to be "embargoed until 2am PT, November 5th".