Pop up: A window, usually an ad, of varying dimensions, that pops on top of your screen
Pop in: an ad that occupies the bottom third of a web page and can be scrolled to be a full page.
Pop Under: an ad, usually full screen size, that appears under the window you are viewing.
From your question I deduce that you are looking to buy cheap bulk traffic. Except for branding purposes, this rarely works.
Of those three, the popins are the ones that tend to bring better results, but there are not enough sites making the space available and so these campaigns run very, very slow. Very, very, very slow!