Google Analytics
Google Analytics (GA) is a service offered by Google that generates detailed statistics about the visits to a website. The product is aimed at marketers as opposed to webmasters and technologists from which the industry of web analytics originally grew. It is the most widely used website statistics service.
The basic service is free of charge and a premium version is available for a fee.
GA can track visitors from all referrers, including search engines, display advertising, pay-per-click networks, e-mail marketing and digital collateral such as links within PDF documents.
Read more about Google Analytics: History, Features, Technology, Performance Concerns, Privacy Issues, Legislation, Support and Training, APIs For Third-party Application Support, Popularity