Quantcast is a web analytic site which offers a better insight on advertising solutions. Basically it is used by advertisers to gauge their audiences and visitors while for publishers, it is used to accurately represent their sites to potential advertisers. However, their main focus are audiences from the US, thus their marketing objective is only for those seeking US visitors. Just like most other web analytic applications, webmasters have to add an code to their site in order for the stats to be updated. Think of them as a web stat ranking site just like Alexa which helps marketers around the world choose sites for advertising.