nomis5 - 11:54 am on Nov 1, 2012 (gmt 0)
Several things actually... for example, you visited a webpage of "carved widgets". You read it long enough and even added the item to the cart, but then abandoned it... BINGO, you've left a trace.
Very true, some ad servers buy ad slots based on just that premise, I have one company doing exactly that and serving some of the ads on my site. The other ads (the leftovers) are served by Adsense. I don't like the idea but without doubt the eCPM is higher compared to Adsense.