Some tools such as domaintools can show "Visitors demographic breakdown" such as "Visitors by Country" data? How are they able to gather this information? thanks
From my understanding, DomainTools grabs data at Alexa, Compete, and Quantcast and displays those stats on the whois record if the domain has enough traffic to show up. Also, you have used external tracking software, ad programs, embedded videos, etc.--some of them collect demographics data and Domain Tools purchases it from them.
I've used Quantcast on several sites, and it seems that the demographic information is pretty much worthless (i.e., entirely incorrect) until I actually include their JavaScript snippet on my pages. My guess is other tools are similarly way off base, so I wouldn't rely on domaintools or any other source that isn't directly measuring the traffic.