Pop-under appears hidden behind the main browser window.
The pop-under is a full screen ad triggered when a Visitor clicks anywhere on the page he visits.
Once displayed, the pop-under remains unnoticed until the main browser window closed or minimized, leaving the visitor's attention free for the advertisement.
A pop-up appears on the main browser window,
The pop-up is a full screen ad triggered when a visitor's clicks anywhere on the page he visits.
It can show the whole contend of ads to the visitors and have a stronger effect.