Windows that open separate from the main window, usually smaller. These separate windows are annoying and often unrequested and containing advertisements. Because the user MUST interact with them, it is thought that people are more likely to read them.
Pop-ups designed to open and then sit behind the main browser Window. These are often even more annoying.
Some pop-ups are caused by adware, which installs in the Web browser, showing advertisements occasionally or everytime the browser visits a new page. While most browsers have developed tools to block out pop-ups or allow some amount of user control, Internet Explorer, by far the most popular browser, has and will be victim to pop-up ads.