This is a great paper defining trading desks, telling you who the big players are and what are the pros and cons of working with a trading desk --- A centralized, service-based organization that serves as a managed service layer, typically on top of a licensed demand-side platform (DSP) and other audience buying technologies; manages programmatic, bid-based media and audience buying. Works as an agency's internal "center of excellence," supporting agency teams wishing to tap into this new buying model on behalf of agency clients.