pop-up ads are a form of online advertising on the World Wide Web intended to increase web traffic or capture email addresses. It works when certain web sites open a new browser window to display ads. The popup window containing an advertisement is usually generated by JavaScript, but you can generate by other means too.

A variation on the pop-up window is the pop-under advertising. This opens a new window behind the active window. Pop-unders interrupt the user less, but not seen until desired windows are closed, making it more difficult for the user to determine which website opened them.

By the early ad-supported Web sites, banner ads were generating enough income, but following the dotcom crisis, the prices paid per click banner advertising declined and many vendors began to investigate methods of publicity effective. pop-up ads, by their nature are difficult to ignore or overlook, and are said to be more effective than static banner ads. Pop-ups have a much higher rate to click on ads to Web Banner (on all popup ads No 14000 is clicked).

Pornographic websites are among the most common users of pop-up ads. Some types particularly vicious pop-up ads (again, most often in connection with adult entertainment sites) appear to have either been programmed incorrectly or has been specifically designed to "hijack" the session of a user Internet. These forms of pop-ups sometimes create multiple windows, and as each window is closed by the user activates the code that generates a new window - sometimes indefinitely. It is sometimes referred to users as a trap for Java "," cascade "spam" or "pop-up Hell", among other names. Usually, the only way of stopping this is to close your browser.

Opera was the first major browser to incorporate tools to block pop-ups, the Mozilla browser later improved on this by blocking only pop-ups generated as the page loads. In the decade of 2000, all major web browsers except Internet Explorer (then the most popular browser and even more from 2006) allows the user to block unwanted pop-ups almost completely.

In 2004, Microsoft released Windows XP Service Pack 2, which added pop-up blocker in Internet Explorer. Many users, however, remain unaware of this ability, or choose not to use it. Many others are not able to use it at all, and not using Windows XP SP2, but earlier versions of Windows. Some users install non-Microsoft ad blocking software in place.