A variation on the pop-up window is the pop-under advertisement. This opens a new browser window, behind the active window. Pop-unders interrupt the user less, but are not seen until the desired windows are closed, making it more difficult for the user to determine which Web site opened them.
For early advertising-supported web sites, banner ads were sufficient revenue generators, but in the wake of the dot com crash, prices paid for banner advertising clickthroughs decreased and many vendors began to investigate more effective advertising methods. Pop-up ads by their nature are difficult to ignore or overlook, and are claimed to be more effective than static banner ads. Pop-ups have a much higher click rate than web banner ads do (about every 14,000th popup ad is clicked on).
Pornographic web sites are among the most common users of pop-up ads. Some particularly vicious types of pop-up ads (again, most often seen in connection with adult entertainment sites) appear to have either been programmed improperly or have been specifically designed to “hijack” a user’s Internet session. These forms of pop-ups sometimes spawn multiple windows, and as each window is closed by the user it activates code that spawns another window — sometimes indefinitely. This is sometimes referred to by users as a “Java trap”, “spam cascade” or “Pop-up Hell” among other names. Usually the only way to stop this is to close the browser.
Opera was the first major browser to incorporate popup-blocking tools; the Mozilla browser later improved on this by blocking only popups generated as the page loads. In the early 2000s, all major web browsers except Internet Explorer (then the most popular browser and still as of 2006) allowed the user to block unwanted pop-ups almost completely.
In 2004, Microsoft released Windows XP SP2, which added pop-up blocking to Internet Explorer. Many users, however, remain unaware of this ability, or else choose not to use it. Many others are not able to use it at all, as they do not use Windows XP SP2, but older versions of Windows. Some users install non-Microsoft ad-blocking software instead.