Definition: Group is a collection of ads. A group is different from a zone. For publishers with many ads, it's better to group related ads together so you can quickly access them. Groups do not have serving code like zones. A group is simply an additional management/classification level for ads. When you open the group page, you'll see the listing of ads linked to this group.
Example: a group named CookingAds for ads related to cooking, which could belong to different zones (for serving purpose)
Relationship: a publisher may have many groups, a group may link to multiple ads and an ad could link to multiple groups.
Ad Group "Publishers with many ads now have an additional layer of ad management and classification. A group is a collection of..." 68% relevant in About AdSpeedWhat's New
What is AdSpeed API? "AdSpeed API (Application Programming Interface) is an add-on service we provide to our clients. Publishers and advertisers can write software..." 24% relevant in Working in AdSpeedTechnical