Currently, when you type just a word in to the address bar, you are taken to a Yahoo! search for that word. For example, I type in "lightning" in the address bar and hit enter and the resulting page is the Yahoo! search results for "lightning."
Some folks might want to have this default to a different search engine. This is very easy to do. Here is how to change the default to Google
* Type about:config in to the address bar
* Search for an option called keyword.URL
* Double-click on the option and change the value to http://www.google.com/search?hl=en&q=