That would be the Google Spybar.
If the PR tool is enabled, every page you visit is tracked. How Google use this data is questionable but they have this data nonetheless. This may be one of the reasons Google fought the DoJ so hard in court recently. If forced to hand over search records, they might be forced to hand over this data too - that would be even more contentious.
Incidentally, if you use the PR extension for Firefox, Google still gets the same data - this is probably the reason that Google tolerates its use.