Advertising accounts are at the top of the Twitter Ads API hierarchy. Together with Twitter user accounts, these two types of accounts are involved in using the Ads API.
Campaigns define the schedule and budget of an ad. The advertiser specifies a daily and overall budget. The campaign can be bound to a specific start and end time or run continuously until the budget is spent.
Funding Instruments comprise the source of the campaign budget. Funding Instruments can not be created via the Ads API, they have to be already established by the advertiser's account manager.
Ad groups, known as line items in the Ads API, exist under campaigns and are used for targeting and bidding against a set of Twitter users. Advertisers promote Tweets or media by associating them with a line item.