What is an ad group in Google Adwords?
An ad group contains one or more ads that share similar targets. Each of your campaigns is made up of one or more ad groups. Use ad groups to organize your ads by a common theme.
Campaigns are made up of one or more ad groups. Ad groups are just that: groups of ads that are targeted to a set of related keywords. This helps you group related keywords and write ads around a common theme. Each ad also includes the landing page you want to take users to after they click.