What is a Pop-Up (or Pop-Under)?
A pop-up ad comprises of a webpage that opens up in a new window in front of a user’s current browsing window. This ad unit has been mostly replaced by the slightly less annoying pop-under which is the same except it appears behind the users current browsing window (or sometimes on a new tab).
These ads are often confused with overlay/floater ads which also “pop-up” but in fact, the terms Pop-Up or Pop-Under only refer to ads that take up whole new browsing windows.
Pop-Up/Under browser windows can be set up to have regular browser window functionality missing such as scroll, navigation and menu bars and can be set to any size. These ads can either contain a bespoke ad of any description (size not being a problem when the window size can be determined by the advertiser) or just be a link to a webpage.
These ads are generally spawned when a new page is opened, but can also be set to spawn when a user clicks anywhere on a page.
What it looks like
Pop-Up/Pop-Under Advice for Site Owners
These ads can command a reasonable eCPM, but again are exceptionally annoying to users so should be limited in the number of appearances they make to a user each day (generally a frequency cap of 1/24 is acceptable).
Most ad networks are capable of running pop-unders but we would strongly recommend against using them as reputable companies on the whole no longer use these sorts of ads. In general, the least problematic of advertisers you will see with these ad types are either dodgy sites or ads for gambling as, so the risks you are taking with your site’s reputation and user count may not outweigh the money you will make.
Also, due to pop-blockers being very common the amount of inventory available for these units is very low (and very hard to predict).
If you do allow them, demand to be paid on a CPM model or else you may be annoying your users for nothing.
Tip: As many pop-ups/unders are webpages with multiple links it may not be possible to track clicks on them (despite claims otherwise from advertisers).
Pop-Up/Pop-Under Advice for Ad buyers
In theory, these ads are the most attractive proposition there is – any sized ad you want or else taking someone directly to your webpage and all for very low CPMs. In reality, however, these ads have become very disreputable in general so they have become cheap to buy because dodgy sites/products use them to advertise on dodgy sites. High-quality sites will in most cases not allow them, and if they do it will be for a very high price which is generally not worth it.
Unless you have a dodgy product/site to advertise and don’t mind where it is advertised it is best to steer clear.