Alexa is a traffic measurement standard and has gained much popularity since the last few years. Alexa is used by big companies to measure the traffic of any website and to calculate a worth of a domain.
Alexa's popular product is their Alexa browser bar, which has millions of users, and each website they visit is recorded by the Alexa bar and sent to Alexa for analysis. The data collected by Alexa bar is then used to show public stats for a domain. Big Advertising companies also use Alexa for deciding either they should accept new publishers or not.
Enter any domain to check its Alexa ranking on real-time.