Sitecore uses the idea of profiles to categorise each visitor. These profiles and how each page affects them are defined by the site owner. As the visitor navigates around the site, their history on the site is used to build up an idea of what sort of customer they could be. Custom events can also be created to categorise visitors based on their actions. If the visitor has purchased something through the site they can be identified as a returning customer. If the visitor has come to the site through a specific Google search query, then that can affect their profile as well.
All of this profiling is good, but where the real power comes from is being able to customise the site based on these profiles. Customers that have shown an interest in the environment can be directed towards the more “green” products. Customers that have already bought products can be directed to buy accessories or service products. So rather than having one “goal” for the site and filtering everyone towards that one goal, different goals can be set up specific to customer types.