Alexa ranks every site on the web based on number of users and number of pageturns.Alexa's traffic rankings are based on the usage patterns of Alexa Toolbar users over a rolling 3 month period.
A site's ranking is based on a combined measure of reach and pageviews. Reach is determined by the number of unique Alexa users who visit a site on a given day. Pageviews are the total number of Alexa user URL requests for a site. However, multiple requests for the same URL on the same day by the same user are counted as a single pageview. The site with the highest combination of users and pageviews is ranked #1. Alexa's traffic rankings are for top level domains only (e.g. domain.com). Alexa do not provide seperate rankings for subpages within a domain (e.g. www.domain.com/subpage.html) or subdomains (e.g. subdomain.domain.com).