The document describes the supply-side platform (SSP) which aggregates digital advertising inventory from multiple publisher sites and networks. It sells this inventory to demand-side platforms (DSPs) which allow agencies and advertisers to buy digital ads across multiple exchanges. SSPs optimize yield for publishers through real-time bidding or preferential rates to buyers in closed networks. Key SSPs include Google, OpenX, PubMatic, and Rubicon Project. DSPs offer advantages over ad networks by charging simple transaction fees rather than markups and allowing centralized buying, serving, and optimization of ads.