Pop-ups, usually referred to as 'OUTS' (4 letters) and 'ADS' (3 letters), are small windows that appear on a computer or mobile device screen. They are often used by websites to display advertisements, notifications, or other information. Pop-ups can also be used to provide additional functionality, such as a search box or a login form. Pop-ups are generally considered to be intrusive and annoying, and many web browsers have built-in features to block them.