Behavioral ... advertising ... is targeted to a specific individual based on that user's previous surfing behavior. This is quite different from the more common targeting method of displaying ads matched to the specific content of an individual page or to all users in general. With behavior targeting, this would mean that two people could see vastly different ads when viewing the identical webpage at the same time.See also my previous posts, "Google wants to change advertising", "Microsoft adLab and targeted ads", and "Yahoo testing ads targeted to behavior".
Studies have shown that conversions are higher when people are targeted through behavior rather than content because behavior can determine a person's actions. Whether it is looking at specific sections of an online newspaper or visiting a certain type of site more than once, those actions are used to determine each user's interests.
See also my previous post, "Is personalized advertising evil?"