This is the summary of an article by Miguel Helft and Tanzina Vega. Here’s the link to the original behavioral targeting article: Retargeting Ads Follow Surfers to Other Sites.
If you remotely show interest in buying a certain product that appears in a website, but decide not to for now, that product can follow you around as ads that appear in other websites. If you don’t know anything about behavioral targeting, it will probably creep you out. While more people are now more familiar with targeted ads, the thing is, these ads are becoming even more targeted and suited to meet individualized needs and highly specified products they have seen online. This technique is known as personalized remarketing or retargeting.