Think of it this way, a supermarket plays an ad for an upmarket beauty product only when it sees an upmarket woman approaching.
Within milliseconds, the cameras pick out the age, gender and mood of the person approaching it with about 95% accuracy. Then, they start to display the promotions that are targeted towards that particular demography.
A good example would be a scene from Minority Report, where Tom Cruise is followed through a shopping center by personalised advertising. It is a bit scary though.