Pop-up ads or pop-ups are a form of online advertising on the Internet intended to attract web traffic or capture email addresses. It works when certain web sites open a new web browser window to display advertisements. The pop-up window containing an advertisement is usually generated by JavaScript, but can be generated by other means as well.

Harmful examples of some pop-ups also claim to be the best anti-virus software that can be found but most of the time can infect the user's PC with malware like Zlob and Vundo. They may also claim that the user's computer is infected by viruses in order to get them to install the rogue.

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