It is important to be clear about the different types of Pop-up so that you choose the one that is right for your needs.
Pop-In – This is similar to a Pop Over, but uses a slightly different method to create the Pop-up.
Pop-Over – A window that appears over the existing browser window. This is the conventional kind of Pop-up and has now been around for several years.
Pop-Under – Works in a similar way to a Pop Over, but appears under the browser window rather than on top of it.
Site Exit – This kind of Pop-up only appears when someone exits the site. It will then appear on top of any other browser windows that are open at the time.