http://www.alexa.com/ Alexa is pretty much the most trusted organisation that use a ranking system to determine how much traffic each website get. You can type yours in the box and it will tell you. Number 1 being the most visited site and so on. Its pretty accurate and for that reason it is well trusted. According to alexa, google is currently number 1 and yahoo number 2. Hope that helps.
If you plan to monetize your site, Alexa rank will be one of the most important factors considered by advertisers
Alexa traffic rank is a service from Amazon that has evolved from a service to those only using the Alexa toolbar to something that is quite likely more useful these days as it it is not restricted to Alexa toolbar users only. Alexa is an Amazon service just as WayBackMachine http://www.archive.org/web/web.php is.
alexa traffic rank is how many visitor you getting everyday so can check that if you download the toolbar but i don;t really care about the alexa rank because doesn't have any affect on the page rank
The Alexa Traffic Rank reflects the number of visitors to a Web site, as well as the number of pages on the site viewed by those visitors based on three months of aggregated traffic data from users of the Alexa Toolbar.