demand-side platform

a.k.a. DSP, not SSP

Traditionally it was a system that allows online advertisers to manage multiple ad campaigns through one interface. Real time bidding for display online ads takes place within the ad exchanges, and by utilizing a DSP, marketers can manage their bids for the banners and the pricing for the data that they are layering on to target their audiences. It is also used for trading desks to grab inventory.

SSP refers to supply-side platform.