Google Analytics (GA) is a free service offered by Google that generates detailed statistics about the visitors to a website. The product is targeted towards marketers rather than webmasters and technologists.
GA can track visitors from all referrers, including search engines, display advertising, pay-per-click networks, email marketing and even digital collateral such as links within PDF documents.
Integrated with AdWords, users can optimize online campaigns by tracking landing page quality and conversions. Goals might include sales, lead generation, viewing a specific page, or downloading a particular file. These can also be monetized. By using GA, marketers can determine which ads are performing, and which are not, providing the information to optimize or cull campaigns.