Retargeting is a term used to describe showing users ads that relate to their recent activity.

For example, when a customer comes to a website, a pixel places an advertising network's cookie on their browser. How much information this cookie contains is subject to privacy agreements. It could contain items from their shopping basket.

The user then visits other websites. Some of these other websites will have ads served by the same advertising network that dropped the cookie. The cookie will then be sent to the advertising network, allowing the network to serve related ads.

In order to be a useful return on effort, this type of retargeting needs to be executed amongst a large network of sites.

Retargeting asserts that people want more of the same. i.e. if they were just browsing for product X, they will will also want to know about related product Y. However, after some period of time if they have already completed the purchase, they probably wont want to know anything about product Y because they have already bought product X. In my case I still get ads targeting a recent bordeaux trip that I have already been on. (booking.com in fact). Do I really want to know about a better room rate now that I've already booked mine?
 
Retargeting may therefore have a shelf life in certain cases. Where as I may still be interested in travel ads, I may not necessarily be interested in bordeaux ads. The ad has outlived the expired buying intent.
 
Retargeting can also be at odds with frequency capping. i.e. limiting the number of times you serve the same ad to the same user.
 
How can retargeting be used in lead generation?