Ad retargeting is when ads you see start to get personalised based on an intent that you started and therefore indicated an interest in – you clicked on an ad for a particular product, maybe added something to a shopping basket, or even just did a product reviews search, and now all the ads you see are for the same damned kind of product. Really clever retargeting works also with abandoned shopping baskets and email follow ups, but mostly it just assumes that because you bought one of a thing, you now want many of them (which might be true for videogames, but not say, for humidifiers).
For example, click on a mattress related advert at a site about mattress warehouse for example and suddenly you will see ads for mattresses on every site you visit (every site using Google for ads, anyway).
The proof of concept website http://retarg.me from the team behind DeepLink is an empty page carrying a bunch of ads delivered by Google and Amazon – we’ve embedded it below in an iframe for convenience. Note that you won’t see the ads if you’re using an ad blocker, and that the ads might not be retargeting you if you have the “Do Not Track” setting turned on in your browser (or more likely, you can see that you are still being tracked even though you have turned that setting on!).