Pop-up ad

From OLSEncyclopedia

Pop-up ads are the scum of the internet. Usually, though, it will be a pop-under instead, meaning the window opens behind your browser window so you don't actually see the ad until you close your browser. This can lead to aggrivation.

Fear not, my friend. Simply turning off Javascript or installing the NoScript extension for Firefox can exterminate these vile creatures from the internet.