As you may have already noticed, each web analytics software, ad network and traffic estimation service (e.g., Compete, Alexa, Quantcast) will report a different number.
Why is this? Because what represents a unique visitor and a page view to your website is subjective (i.e., each service and software has its own criteria for deciding when a visit and a page load happens). For example, some services and software count search bots as visitors, whilst others don’t.
1. Raw logs
Most servers store raw logs, which are lists of all the accesses and page requests on your website. It’s possible to interpret those raw logs with special programs, creating graphic reports which will contain the number of visitors, page views and so on.
Those numbers are grossly overestimated, though, because all kinds of search bots and automated queries are counted together.
2. Webalizer and AW Stats overestimate
Webalizer and AW Stats are very popular web analytics programs, and that is because they are usually installed by default on cPanel (the control panel software on most hosting companies). Both of them tend to overestimate the number of visitors and page views your website receives, however, and such data should always be used with a grain of salt.
3. Ad networks underestimate a bit, but there is nothing you can do about it
The number of impressions you’ll see on most ad networks control panel usually is an underestimation of your total traffic, and that is because they won’t track people who can’t see ads or who block them on purpose.
There is nothing you can do about it though, and if you want to make money using ad networks you need to play under their rules. The alternative is to have your own banners embed with HTML code, in which case they would be seen by 100% of your visitors.
4. Google Analytics underestimates a bit, but it’s the industry standard
The numbers reported by Google Analytics also underestimate your traffic slightly, and that is because the software has very strict rules regarding what should be considered a visitors and a page view.
Google Analytic's tracking is very reliable, though, and that is why it’s used as the industry standard. If you want to sell a website, for example, most serious buyers will ask for Google Analytics data before they make an offer. To sum up: Go with Google Analytics if you want to get a sense of your “real” traffic.