Did you ever notice that no matter which site go you keep seeing the same advertisements? Well, it happened to many times. So I was kind thinking wow what's wrong with this ad? Is it following me or something? Then I came across this video and found out, yes some ads do follow you where ever you go and it's called Remarketing.
It's actually an option for advertisers who want to grab those people who once came to their site but didn't buy anything, so they want go after them to convince them to come back. LOL In my view point, it seems like those advertisers do this are like those cheap vendors who keep trying to sell you something even though you don't want it. And then they keeps following you where ever you go.
I don't know why advertisers even want to go for this cheap way of targeting people. I think many times instead of going back if he (visitor) finds then most likely he will be pissed off. Watch the video from 7:57 for more explanations.