Google.com - The leading search engine on the internet today with approximately 80% of all search traffic. When people speak of search engine optimization (SEO), they're often referring specifically to Google.
Googlebot - The crawler that Google uses on a daily basis to find and index new web pages.
Google Toolbar - A downloadable toolbar for Internet Explorer that allows a user to do a Google search without visiting the Google website. The toolbar also displays the Google PageRank (PR) of the page currently displayed in the browser. The latest version also includes a very good popup-blocker. The Google Toolbar is a must have for every serious webmaster.
The Google Toolbar can be downloaded here:
http://toolbar.google.com