I know the answer to this is buried in here somewhere, but the acronym is too short to run a search on.
I'm guessing it stands for interactive banner, a.k.a. a JavaScript banner or something similar?
International Advertising Bureau. (http://www.iab.net/)
They in a nutshell set advertising standards such as ad dimensions, types and practices.
Commonly, when people talk about IABs, they are talking about specific banners e.g. leaderboards, skyscrapers, etc.

Thank you for the explanation!