IAB stands for the Interactive Advertising Bureau (or Internet Advertising Bureau in the UK). They are a trade body for online advertising who have helped define online advertising since 1996.
Most notably they designated a set of standard ad sizes which helped display marketing take off by reducing technical issues between publishers and advertisers. These standard ad sizes (728×90, 300×250 etc) are still in use today.
The IAB also came up with the VAST (Video Ad Serving Template) standards, which made it possible to serve ads across many sites.