Adding an Advertiser (for instance; CocaCola) is a natural first step when a salesperson has closed a sale with a new buyer. An advertiser can contain multiple orders.
Determines who has access to the campaign, and what set of sites that impressions can be delivered to (see Teams for more information). One order can contain multiple line items.
Determines start and end dates, delivery objectives (impressions, clicks or conversions), pricing, targeting, creative delivery and prioritization. One line item can contain multiple creatives.
The material shown to the end user. Can consist of various assets such as images, text and videos.
An analysis you can run to forecast the volume of matching traffic for a line item.
Here is how the various objects hang together: an advertiser is the top level object, and contain multiple orders, which in turn can contain multiple line items, which in turn can contain multiple creatives. Creatives will then be served inside ad units, which are set up under the inventory section.