Normally we do is enter some data to search in the textbox and click on "Google Search" button, which shows a listed of matching websites in page wise.
I'm Feeling Lucky :
If we enter some data to search in the textbox and click on "I'm Feeling Lucky" button, it will directly takes you to the first website. First website means first site shown in the list if it is normally searched.
For more information:
http://google.about.com/od/searchingtheweb/qt/imfeelingluckyq.htm
