An agency trading desk is a department or subsidiary of an advertising or media agency. It is often considered as an "internal" center of excellence for managing digital advertising buying on behalf of agency clients. For more details about characteristics and missions of a trading desk, see trading desk definition.
Agency trading desk are sometimes criticized for the classic "independance debat" and for double paying - the agency and the related trading desk are both paid.
It has to be noted that agency trading desk are often discreet about their family relationship.
Xaxis trading desk, a subsidiary of WPP: