Android admob ads ruin layout?

I am fairly new to android dev, and I am looking at implementing android admob ads into my app. I have never implemented ads before, so I was wondering if the ad squashes the layout on the screen or if it simply overlays over the other items such as im开发者_运维问答ageviews etc. I am worried because I would like an ad on the bottom and if it doesnt overlay it will screw up the whole positioning of my pages. Would anyone know the answer? Thanks.

As a user, I have seen admob ads which overlay the screen.

_{I swear at them every time they block the corner of the screen I need to see in Angry Birds.}