I have no idea what algorithms/criteria they use, but I'm sure it's like Google's AdSense where they track where people go and make *assumptions* from that information.
Not too long back Google, for instance, started what they call 'visitor interest' ads or something to that effect. They used to serve ads based upon the content (as best they can tell) of each web page. They still do that, but in addition they serve ads that their tracking indicates a person is interested in. Geo-location targeting is also used so someone visiting here from, say, Britain will see different ads than someone from the US.
I'm not a person who worries about 'privacy' as to places I browse on the internet, but I know some people don't like that aspect of the internet these days. Bottom line is unless one wants to go through some pains to use an anonymizer or Tor, they will be tracked. Even if one does use an anonymizer or Tor I'm not convinced tracking isn't possible.
Just like having a cell phone - Unless the battery is removed you have to *assume* that your cell phone is leaving a train of where you've been. In fact, there was recently a study that showed that using cell phone data people's movements can be predicted to a high degree. In a study published in Science, researchers examined customer location data culled from cellular service providers. By looking at how customers moved around, the authors of the study found that it may be possible to predict human movement patterns and location up to 93 percent of the time.
The times, they are a changin'.
