DSPs connect to ad exchanges and Supply-Side Platforms (SSPs) to access available inventory, then apply an advertiser's targeting rules, budget, and bid strategy to decide which impressions to buy and at what price — all within the roughly 100-millisecond auction window before a page finishes loading.
Modern DSPs typically support cross-channel buying (display, video, connected TV, audio, digital out-of-home) from one platform, along with first- and third-party audience data integration, frequency capping, and brand-safety controls.