Alexa Internet, Inc. is a California-based subsidiary company of Amazon.com that is known for its toolbar and Web site. Once installed, the toolbar collects data on browsing behavior which is transmitted to the Web site where it is stored and analyzed and is the basis for the company's Web traffic reporting. Alexa ranks sites based on tracking information of users of its Alexa Toolbar for Internet Explorer and Firefox and from their extension for Chrome.
About (from Alexa.com at www.alexa.com)
“Alexa is the leading provider of free, global web metrics. Search Alexa to discover the most successful sites on the web by keyword, category, or country. Use our analytics for competitive analysis, benchmarking, market research, or business developm”