They call it Expired Domains Traffic. They are most of the times a disservice for the internet users, but I'm linking to them for public interest.
The idea is quite simple. They have a bot searching for the expiration of domains. When a domain expire, they buy it, and them make it redirect the traffic to their customers site.
What's the point there? Let them explain it:
About "Expired DomTraffic"
Every day 1000s of previously registered domains expire, because the owner did not extend domain registration . If the owner does not pay the annual fee, the domain registrar will put the name on hold. With most registrars, an "on hold" domain stops working. Most registrars allow an additional grace period of 30-90 days for the domain owner to pay the annual fee. During this period, the registrar will generally contact the domain owner many times with attempts to get them to pay the fee and reactivate the domain name. If the domain owner fails to pay on time, and fails to respond during the 30-90 day hold period, the registrar will drop the domain name. At this point, anyone can register the name. We assume that the previous owner no longer wants a dropped name and we will register the name if we feel that it will generate traffic. After we own the name, we direct it to our server and send out the expired domain traffic (= guaranteed visitors) to the campaigns we serve.
These expired domains have traffic on them, and the previous owners marketed them so you now reap their hard work by having us direct this traffic to your website. This type of redirection is better than popups or popunders. AOL browsers now completely block pop-ups and about 60% of all internet surfers have installed pop-up blocker software anyway.This is why expired domain traffic will generate much better results than pop-up or pop-unders!
It's interesting to have a glimpse on how things really work on (under) the internet.