Both pop-up and pop-under ads can be classified as pop-ads. Functionally, they are both pretty similar: they both open a new browser window, usually via some sort of JavaScript. The difference between them is what happens with that new window once it’s opened. A pop-up ad will launch a new browser window on top of the one currently being viewed. The main issue with this is that it can be seen as being quite intrusive.

This is where pop-under ads come in handy. Rather than being displayed in front, the new window opens behind the one that is currently being viewed. It does not interrupt the user experience. If a user has many windows open, when they’re done with the page they were viewing at first and close it, if they come across a pop-under ad they are less likely to know where that page came from. This means they are also less likely to have a negative emotion about the page that loaded the pop-under.