Pop-under is a type of window that comes behind the browser window of a web page what you are visiting. There is a difference between pop-ups and pop-under because pop-up ads always appears over the browser window but pop-under is less disturbing than pop-ups because it hides behind other windows and pop-under has other objectives also than advertising.
There are some pop-unders that are very useful. Sometimes you might see pop-ups which ask information from you or give you information. Sometimes pop-unders are used in music website so that it can play music by opening a small window. There are also unwanted pop-under ads also. It is the programming of JavaScript that maintain the size of pop-under and decide which element will be displayed. Sometimes you will find very difficult to trace the menu or status bars of the pop-unders if they are hidden. The only option is click the “x” above corner side. And this is very irritating.
Some pop-under has problem because the JavaScript is managed very poorly there. It causes to open several windows at the same time and same place. Then exit from the browser is the only option for you.