Line item templates lets you add targeting, pricing, frequency capping and much more to a template, that can be re-used whenever you create a campaign that should inherit those settings from your template.
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.
The hierarchy of the objects under Advertising, and how the connect to ad units.