If you dig into their sites, there are some general descriptions. My understanding is that they partner with ISPs, toolbars and the like to get data on people's browsing habits.<p>So small time ISP might share what sites are being visited, so they extrapolate on that. Alexa uses a toolbar, for example. So they know what sites people are browsing.<p>Quantcast lets you add your own codes, so then you get an actual measurement of traffic.