Demand-Side Platforms (DSPs) are the programmatic execution platforms that marketers use to buy display, video, native, and emerging-channel ad inventory across the open web, mobile apps, and increasingly retail-media interfaces. Core capabilities include real-time bidding on programmatic exchanges, audience targeting via first-party data and third-party segments, brand-safety controls, frequency capping across publishers, creative management for sized banner units, and reporting on viewability, engagement, and conversion. The market spans the dominant independent DSPs (The Trade Desk, DV360, Yahoo DSP, MediaMath, StackAdapt, Basis Technologies, Simpli.fi), agency-flavored execution platforms, and B2B-flavored programmatic (AdRoll).