Google Analytics is the tracking software used to discover who is visiting your website. Google Analytics can tell you where the hits are coming from, how long they are on your site, what pages they visited and i think most important- how they got to your site. I can see who is linking to me from where. This service has an huge value, previously companies would charge thousands a year for this service that google now offers for free. So if you have a website this is a must have to learn more about your visitors.