Ad Server a system that ensures a creative is delivered to a user client, which may also include dynamically optimizing the creative. The ad server may also track delivery and user engagement metrics associated with the creative (e.g., impressions, clicks, actions, video completed view).
Publishers rely on a publisher ad server to control what ad shows in each ad slot on their web properties. Publishers with sales teams sell direct deals to buyers that often specify how much inventory must be delivered by a given date. Publishers of all sizes often rely on direct sources of demand to "fill" and monetize ad slots on their web property.