A pop-up is a window of the browser you use. Pop-ups appear when you download some sites and often contain advertisements. As a rule pop-ups do not have common panels typical of browser windows, they often are full-screen size and do not have the upper right corner X to close them. Usually pop-ups are harmless to your computer information but their codes may contain different malicious functions such as exploits. Pop-ups are a kind of spam and distract your attention from the main information making you close unnecessary windows and create simple irritation.