[lawyertrust]

The Alexa rank is a ranking system calculated using the Alexa toolbar. Alexa takes the browsing data from the millions of people using this toolbar and calculates the traffic ranking of each site based on reach and page views. Low traffic sites are harder to gauge accurately, but once a site reaches a 100,000 or less ranking the accuracy of the measure increases.
Hence the lower your Alexa rank, the credibility of that ranking is increased. The top ranked website, Google has an Alexa rating of 1, Facebook is #2. Less popular sites could be ranked well in the millions.

[antikeparusto]

As a first step, Alexa computes the reach and number of page views

[kirbie]

Alexa ranking is a tool that Alexa company uses to collect data on websites and publishes the ranking it.