Pop-up ads or pop-ups are a form of online advertising on the World Wide Web intended to attract web traffic or capture email addresses. As pop-up ads became widespread, many users learned to immediately close the popup ads that appeared over a site without looking at them.
Pop-under does not interrupt the user immediately and are not seen until the covering window is closed, making it more difficult to determine which web site opened them. Opera was the first major browser to incorporate tools the Mozilla browser later improved on this by blocking only pop-ups generated as the page loads.
In the, all major web browsers except Internet Explorer allowed the user to block unwanted pop-ups almost completely. Pop-under ads are similar to pop-up ads, but the ad window appears hidden behind the main browser window rather than superimposed in front of it.