The Google toolbar is an add-in toolbar for Internet Explorer. The toolbar is snapable just like the Address and Links toolbar in Internet Explorer, and contains a Google menu with quick links to the Google home page, Google Web Directory, and other Google-related pages. And of course, the page contains a search box. Another handy feature is highlight which highlights your search words in the web page you are viewing.
A controversial feature of the Google toolbar is PageRank. PageRank tracks the web pages you visit to rank the popularity of the website. Google terms this as an Advanced Feature and expressly tells you that they are tracking the web pages you visit, but this still causes privacy concerns among some circles. Another advanced feature is Page Info which among other things, allows you to see pages that Google decides are related to the current page.
The Google toolbar can be downloaded from <http://toolbar.google.com/>.