In web design, a pop-up is a graphical user interface (GUI) element that appears on top of the current web page, usually in a separate window or dialog box. Pop-ups can contain a variety of content such as advertisements, forms, login prompts, notifications and messages. They can be triggered by user actions such as clicking a button, hovering over an image, or scrolling down a page. Pop-ups can be either modal or non-modal (see also modal window). Modal pop-ups require the user to interact with them before returning to the main content, while non-modal pop-ups can be dismissed without interaction. While pop-ups can be useful for conveying information or prompting user actions, they can also be intrusive and annoying if overused or poorly designed. As a result, many web browsers now include built-in pop-up blockers to prevent them from appearing.