Every ad campaign has three levels: campaign, ad group and ad. Campaigns house ad groups, and each ad group contains a collection of ads.
Campaigns are the highest level of the campaign hierarchy. At the campaign level, you can select a campaign objective and set an optional campaign spend limit. Each campaign can contain more than one ad group.
At the ad group level, you can choose where your ads will appear on Pinterest and set the budget, bid, run dates and targeting for all the ads housed within them. Ad groups let you address the different regions, product lines and target audiences that may be a part of a single campaign. They also allow you to test the performance of various targeting and objective strategies without creating multiple campaigns.
Ads are the Pins people see. Each ad sits within an ad group and has a unique URL and creative. You can create multiple ads in a single ad group to test which variations in creative work best with your targeted audience.