Quantcast is a Web audience measurement tool that collects and organizes real-time data about the amount of traffic a website is getting, as well as the demographics, psychographics and other Web analytical data about that website's visitors.
Quantcast provides detailed statistical information about a website's audience for website owners, advertisers and publishers, which helps them to make business decisions.
Techopedia explains Quantcast
Quantcast delivers website traffic statistics, which are designed to aid in analyzing the performance of a website from a marketing perspective, as well as to identify buyer personas in terms of their location, behaviors, and buying and behavioral preferences. Websites that are able to capitalize on this data stand to attract more visitors and generate better revenues than websites that don't know their audience.