A pop-up ad is a pop-up window used for advertising. When the program is initiated by some user action, such as a mouse click or a mouseover , a window containing an offer for some product or service appears in the foreground of the visual interface. Like all pop-ups, a pop-up ad is smaller than the background interface - windows that fill the user interface are called replacement interfaces - and usually resembles a small browser window with only the close, minimize, and maximize options at the top. A variation on the pop-up ad, the pop-under , is a window that loads behind the Web page that you're viewing, only to appear when you leave that Web site.
Pop-up ads are not popular with the average Web surfer, and there are several products that disable them, such as Pop-up Stopper, Pop-up Killer, and Pop-up Annihilator. One thing to look for in such a program is the ability to differentiate between user-initiated pop-up windows and others, because many other applications (such as Webcast s, for example) make use of pop-up windows. If a pop-up stopper utility can't tell the difference between a pop-up window that the user has requested and an unsought pop-up ad, the program may cause more problems for the user than it solves.