Here's a good overview.

Web analytics
http://en.wikipedia.org/wiki/Web_analytics

"Web analytics is the measurement, collection, analysis and reporting of internet data for purposes of understanding and optimizing web usage.[1] It is often done without the permission or knowledge of the user, in which case — particularly with third party cookies which can be shared between different web sites — it can be a breach of privacy.

Web analytics is not just a tool for measuring web traffic but can be used as a tool for business and market research, and to assess and improve the effectiveness of a web site. Web analytics applications can also help companies measure the results of traditional print or broadcast advertising campaigns. It helps one to estimate how traffic to a website changes after the launch of a new advertising campaign. Web analytics provides information about the number of visitors to a website and the number of page views. It helps gauge traffic and popularity trends which is useful for market research.

There are two categories of web analytics; off-site and on-site web analytics.
Off-site web analytics refers to web measurement and analysis regardless of whether you own or maintain a website. It includes the measurement of a website's potential audience (opportunity), share of voice (visibility), and buzz (comments) that is happening on the Internet as a whole.

On-site web analytics measure a visitor's behavior once on your website. This includes its drivers and conversions; for example, the degree to which different landing pages are associated with online purchases. On-site web analytics measures the performance of your website in a commercial context. This data is typically compared against key performance indicators for performance, and used to improve a web site or marketing campaign's audience response. Google Analytics is the most widely-used on-site web analytics service; although new tools are emerging that provide additional layers of information, including heat maps and session replay.

Historically, web analytics has referred to on-site visitor measurement. However in recent years this has blurred, mainly because vendors are producing tools that span both categories."