I noticed that. They may be moneymakers buy IMO they are only slightly less evil than popups.
What they are -- if you visit a site doing popunders and then close your browser and wonder where that window advertising something on your monitor CAME FROM -- That my friend is a popunder. Never, under any circumstances clink anywhere in one of these except the X in the upper right corner or better yet right click and choose close.
These are really evil things. Even though the clicks and $$$ are good there I may move all my domains away.