Google's DoubleClick for Publishers (DFP) API lets developers build applications that interact directly with Google's next-generation display advertising platform. publishers and third parties can manage inventory, create orders, pull reports, . Here are some possible applications you could build with the API:

• Create a self-service ad ordering page on your site where advertisers can come to create an order and upload creatives.
• Integrate your content management system with your inventory structure so updates in your content management system update your inventory.
• Connect your billing system with our reporting data to automate invoicing of your advertisers.

 in this articles i will Re-post what google say about it.
ReportService
ReportService enables you to run the same reports seen in the DFP user interface, as well as create custom reports that can be run programmatically to analyze the performance of a network on multiple dimensions and attributes. google provide examples for running three typical reports (delivery, inventory, and sales) that many users of DFP Small Business (and former GAM customers) should be familiar with.