Individual shopping campaigns are structured internally on two levels: Product Groups, and Ad Groups.
Product Groups provide a mechanism to group together similar products within the feed. You can create product groups using various attributes present in the feed. Typically this is used to bid similar products and product variants at the Product Group level.
Each product group is a subset of the feed. The bids are then controlled at the product group level. This can save a great deal of time while managing campaigns with sizeable feeds. When benefitical we utilize ad groups with ad-group level negative keywords to influence keyphrase matching.