A demand-side platform is software that buys ads automatically. Advertisers use it to bid on impressions across multiple exchanges and networks from one interface. Set the audience, the budget, and the bid rules, and the platform handles the rest.

DSPs replaced manual insertion orders for much of digital display, video, and mobile. They make it possible to reach highly specific audiences and to shift spending in real time based on performance.

What a DSP does

• Connects to many ad exchanges
• Applies targeting and frequency rules
• Bids in real-time auctions
• Reports on delivery and outcomes

The mirror image is the supply-side platform, which represents publishers. The two meet in the ad exchange. Transparency remains a running complaint, since fees and placements can be hard to trace through the chain.