A group of ads within a Campaign that are focused on a set of keyword phrases.

Ad Groups are a component of the Google AdWords system. Many users set up their AdWords programs so that the Ad Group is the most precise entity for which there is a unique trackable Query String. However, it IS possible to have higher precision, i.e. keyterm by keyterm tracking without making separate Ad Groups for each keyterm. This is done by using a technique called Google's Power Posting. Power Posting is free and relatively easy to implement.

a collection of ads

A retailers' consortium that advertises together and shares costs.