Alexa's traffic ranking algorithm is primarily based on tracking a sample set of internet users that have the Alexa toolbar installed in their browsers (over 25,000 different browser extensions). The Alexa Toolbar includes a popup blocker, a search box, links to Amazon.com and the Alexa homepage, and the Alexa ranking of the site that the user is visiting. It also allows the user to rate the site and view links to external, relevant sites. To measure how a website is performing relative to all other sites on the web, Alexa calculates the rank, using a proprietary methodology that combines a site's estimated average reach and its estimated number of page-views over the past 3 months. The reach refers to the number of of daily unique visitors with the Alexa toolbar installed in their browsers and Page view, as its name shows, is the number of times a particular page is viewed by the users, however a particular page visited by a user for multiple times a day, is counted as one. In addition to a toolbar, Alexa gathers traffic data from direct sources in the form of sites that have chosen to install the Alexa script on their site and certify their metrics. However, site owners can always choose to keep their certified metrics private. To know more about Alexa click here.