When someone visits a website, their computer or other web-connected device communicates with the website's server. Each page on the web is made up of dozens of distinct files. The site's server transmits each file to user browsers where they are assembled and formed into a cumulative piece with graphics and text. Every file sent represents a single “hit”, so a single page viewing can result in numerous hits.
Websites produce traffic rankings and statistics based on those people who access the sites while using their toolbars and other means of online measurements. The difficulty with this is that it does not look at the complete traffic picture for a site. Large sites usually hire the services of companies such as the Nielsen NetRatings or Quantcast, but their reports are available only by subscription.