A Pop-Up is a visual element of a website that's displayed when the user opens or closes a website or on certain commands

Pop-ups can have multiple purposes. Mostly they are used for advertising but they can also be used to e.g. open an image after the user clicked on a thumbnail.

When used for advertising, the pop-up is usually displayed on opening or closing a website.

The pop-up can be displayed either on top of the main window or in the background.

Many users use a pop-up blocker to block undesired pop-ups. These blockers however don't always work with new pop-ups that were written using JavaScript.