At a recent breakout session I attended on digital marketing by Aidan Augustin and Jeff Bunkin of Feathr was focused on retargeting, geofencing and lookalikes.
Retargeting - someone visits a website and you get an ad later when you're on a different web pages (Facebook, papers, etc. - banner ads or a video ad). This is all about getting you to purchase a product once you've left a site where you were browsing.
Retargeting is all about paying to reach a warm audience and you don't need their email. They've just visited your website.
You can use segmentation to dig deeper in what they viewed on your website and then you can decide what ads they may get from you to make sure you're delivering relevant information to them - think an exhibitor versus an attendee and giving them the right information to sign-up.
Another example is the shopping cart abandonment, where you can retarget them with the registration process and get them to check-out and finish the process.
An ad exchange is an auction site to decide what ad is shown at the higher price on the webpage that is being loaded in real time.
Geofencing - showing ads show within a specific geographic area, think a 4 block radius.
For a great resource on the specifics of retargeting go HERE for an article by Social Media Examiner.