Number of Visitors to a given website can be tracked in many different ways. In case you would like to track number of visitors coming to your website, you have many different analytics tools available at your disposal. Google Analytics is one such website traffic anaysis tool and yes it is free to use. This website also uses google analytics to track number of visitors. Google analytics also offer many other useful analytics tools so that website owener or webmaster can get an idea of from which country people are visiting the website, which browser they are using, what is the screen resolution of the visitors, how much time they spend on the website and many other useful analytic reports.
In case you would like to know how many visitors are visiting a website which you do not own, then the story is little bit different. Google Analytics data is protected with a userid and password authentication of a google account. The Webmaster or website owner of a website can voluntarily show you the google analytics report. Sharing of google analytics report to share data about website traffic is normally done by websites which accept other website’s advertisement or promote products or services of some other website. Another post in this website titled How Websites Make Money tells more about such websites.
However if you would like to know traffic analysis of some website which do not share their data, then you will have to get help of other tools. Various websites such as compete.com, alexa.com, google, and other websites provide a rough estimate of traffic anaysis of a website. This website will cover few examples on the topic so that you can also know how to analyse traffic details of a website which does not wish to share their website analysis data.