Types of line Items in DFP:

Sponsorship: A sponsorship line item is high priority line item sold on the basis of (CPM) Cost Per Thousand/Mile impressions, (CPC) Cost Per Click and (CPD) Cost Per Day. These line items are always goal based and served before other line item, means if have set 100% goal to one line item and no other line item target the same inventory/Goal then the sponsorship line item targeting 100% goal will be served and if you set lesser priority then 100% than the lower priority item will be served along with sponsorship.

Standard: Standard line items are most commonly used for campaigns that have fixed amount of quantity of impressions agreed with client or advertiser, in such line items you have to enter certain amount of impressions targeting a to specific period of time.

Network:Network line item is used to traffic your ads network and allows to allocate the remaining percentage of impressions to serve. It is an important line item because when you refresh the browser, it refreshes the entire page but fail to upload the creative/ads of other line items i.e standard etc, and shows it blank. But if we put creative on network (this can be a similar creative/ad or any other creative) it display there after the user will refresh the browser. So it helps as an alternative image which always be there to fill the blank space of any line item, in case the other does not comes up so there should be always one creative on Network. Network line item is also a goal based line item, we cant not provide the quantity of impressions to it. Also, it can not forecast the results of line item.

Bulk: Line item type is used to saturate site's left inventory or impressions. It works same as standard line item, we can provide specific amount of impressions to it but cant forecast the results of line item.

Price Priority: Price priority line item is used to saturate site's unsold or be left inventory with the highest paying creative/line item. It allows you to state and identify delivery caps. we can set any quantity of impressions for such line item, but can't forecast the results.

For more help, regarding how to create line items and detail in depth please visit- https://support.google.com/dfp_premium/topic/28245?hl=en&ref_topic=4388125