Pop-ups are a form of online advertising used to attract web traffic or capture email addresses. Pop-ups are web browser windows that open on top of the active browser window, often used to display advertisements, special offers, or other ancillary information.
Pop-unders are a variation on the pop-up window, typically an advertisement, which opens a new browser window that is hidden under the active window. Pop-unders do not interrupt the user immediately and are not seen until the active window is closed, but this makes it more difficult to determine which web site opened them.
