Pop-ups are advertisements which appear in separate windows while a user tries to access a regular web page. Pop-ups are usually displayed in smaller browsers and generally make use of JavaScript.

While most pop-ups are ads, some have different purposes. Some pop-ups aim to help users by providing additional information, particularly while a user is filling up an online form. These are generally allowed by browsers which have been set to block pop-ups created for advertising purposes.

Since they are more difficult to ignore, pop-ups are usually more costly than regular advertisements posted on websites. However, this visibility does not always have a positive effect on consumers, because pop-ups are usually irritating for many people. On the other hand, pop-unders become visible to the user only after the main window is closed. If several windows were opened while surfing, the user might not be able to trace the site which caused the pop-under to appear.

Most pop-ups disappear completely once the browser is closed by the user. However, there are some pop-ups which trigger the appearance of even more browsers when the user tries to close them. This can cause even more irritation to users whose browsing activities may be momentarily disrupted.

In response to the browsing preferences of users, Internet browsers have introduced pop-up blockers. Users are given the option to either block or allow pop-ups. Ad filters also perform the same function. Some advertisers have found ways to go around these filters, though. Instead of a pop-up, an advertisement may hover over the actual webpage. If the user clicks on the ad with the intention of closing it, but makes a mistake and does not hit the correct button, he will be directed to the website being advertised.