Google Analytics tracks and reports website traffic. Google launched the service in November 2005 after acquiring Urchin. Analytics is not difficult to setup but it is a very immersive application. A lot of time has to be spent analyzing the demographics of your visitors and where they are going.
There is basic user data and basic information on views. The challenge comes into play when you want to track specific information like product/service conversion. Reporting is straight forward unless you want to delve into custom reports.