Alexa is a service acquired by Amazon which offers a web traffic report for websites. They retrieve the data from toolbar that can be installed in different browsers, centralize the data and display reports to anyone. The most important indicator is the Alexa Rank. It represents the rank of a webpage in a list of all the websites. It’s not the 100% accurate but it gives a good indication.
Alexa does not offer any free API to obtain Alexa Rank. However there is a simple method to obtain it in the same way the Alexa Toolbar does. All you have to do is to invoke the following url(replacing php-html.net with your domain): http://data.alexa.com/data?cli=10&dat=s&url=php-html.net