The structure that contains one or more ads and a target set of keywords for Google Search, and a target set of placements for Google Display. Its is a collection of one or more PPC ads that all target the same set of keywords. Ad groups are contained within a campaign, an Ad Group All PPC ads are required to be part of an Ad Group.
An ad group contains one or more ads which target a shared set of keywords. Click To TweetHere is how adgroups fit in the overall Google AdWords account structure
Here is from Microsoft Bing’s perspective.
