Alexa like trafic rating

Hello

Can anyone tell me how Alexa like trafic rating tool know the trafic to any site.
For visitor counter we need to use counter. But they dont need anything. How? Any comments.
Thanks!

I am pretty sure it's based on people that have downloaded the Alexa toolbar and Alexa is then tracking.

Thanks for you reply.

In this way it cannot be accurate.
Is there any method using which anyone can get the real trafic to any site.

Thank you!

Actually, yes it does:

How does Alexa determine ranking? Alexa isn't entirely accurate, because it relies on statistical sampling that isn't random. Alexa achieves its ranking system by counting how many pages were visited by people who have downloaded their toolbar. And that group of people is anything but random: they tend to be webmasters, internet marketers, and other market-minded people. And technical people tend to completely turn up their noses at Alexa rankings, because installing a toolbar that "spies" on their surfing habits makes them upset, even though Alexa only collects aggregate information (nothing that should be linked back to an individual).

When you need to quickly find unbiased, third-party information about any site on the internet, try the easy-to-use tools that you’ll find at Alexa.com. While its not perfect, it’s a quick way to catch a small snapshot of internet marketing information. For example, if you're considering advertising with a site, check their ranking number to get an idea of how effective they might be at delivering high quality traffic to your website.