A popup window is a graphical user interface element that appears on top of the main content of a website or application. It can be triggered by user actions, such as clicking a link or button, and is often used to display additional information, advertisements, or forms without navigating away from the current page. Popup windows can be either modal or non-modal. A modal popup requires users to interact with it before they can return to the main content, while a non-modal popup allows users to continue interacting with the main content. Common uses include notifications, sign-up forms, and promotional offers.