Start by using your web browser to search. The search results are displayed just like when you use Google, only this time, it will pull files from your hard drive. At the bottom of the search engine, you can see how many files it pulled up.
You can also search with a pop-up window that appears when you double-tap the command key. From there, type the first few letters of the file, application, or element you want to find, and Google Desktop will start listing everything that begins with those letters.
Google Desktop is a useful way to make a copy of previous versions of a file, too. Every time you view a file, Google Desktop takes a snapshot and saves it to a cache on your hard drive.