How many hits does a website get over time? Whether to measure traffic on your own site, see how your competitors are doing this year, or just to observe statistics about how certain websites perform, this question is asked often but can be difficult to answer.
Fortunately, there are many services that help determine the popularity and usage statistics of websites, and if you own or manage a site, there are very powerful tools available that will log and monitor your usage statistics and serve as a starting point for understanding why and where users come from. See this list of traffic tracking tools for both paid and free analytics solutions. Many website hosting companies also provide some default tools for free; it's worth calling to find out what may already be available.
To find statistics on other websites - say a site you like or a specific competitor - you'll have to rely on Internet-wide monitoring services, as the tools for your own website cannot track what people are doing elsewhere on the web. Some of the services that do this are Alexa, Compete, comScore Media Matrix, Hitwise and Nielsen/NetRatings, though only Alexa and Compete are readily available for free.
These services typically gather information by tracking the online behavior of a statistical sample of the internet population. The more users they track, the more accurate their information is, though there is some controversy as to how representative those users are. Regardless of this, these services can only provide estimates of visitor and hit numbers, as the exact numbers can only be tracked by those who own the websites, and many companies keep such information very private.
Also keep in mind that the measurements they provide are generally in terms of percentages of the entire Internet population, rather than a head count of website visitors. This occurs due to the structure of web browsing, which makes it difficult to accurately determine whether someone is visiting a site multiple times, by accident, or is even paying attention to what is on their screen.