Each ad has two elements. Format (what it looks like) and placement (where it appears).
Creatives are ads that are delivered to users in digital environments such as web pages and apps. Creatives include images, videos, audio, and other formats that are delivered to users. To serve a creative, you need to add it to a line item. You can also add it to your creative library for later association with line items.
Creatives are always associated with advertisers.