Popup or Pop-up window appears when an applet or a script forces the browser to open another window.

Pop-ups are normally some kind of unwanted and undesirable advertisements.

Pop-ups are primarily of three types:

• First type of pop-ups is the one that appear when the user clicks on some kind of link on a web page.
• Second are the ones that appear randomly.
• Finally, the third type of pop-up is the one that appear without any user intervention.

The 3rd type is the most annoying kind and display unwanted and often explicit ads containing content that the user would have no liking in whatsoever.

There are also pop-ups that could bring along with them a program that is malicious such as a virus or a worm or a Trojan horse.

These pop-up serve some kind of a security alert and more than often display warnings related to some security breach of your system or some kind of an offer that would scan your system for malware.

If you click in any part of the window, the code is set such that it will download undesirable and unwanted content and infecting your system with malware.

Legitimate companies were also known to use pop-ups for desirable content but their use is diminishing as the ill content malware based pop-ups have become prevalent and user are annoyed with any kind of pop-ups that might appear.