You can buy your ad inventory on the open exchange, or a private ad exchange. Most private exchanges are owned by premium publishers or vertical ad networks. It's important your know the difference.
Advertising business has used a wall street business model to create a fairer market place for advertisers to find the best value for their ads and exercise more power over where they place their ads. Treading desks were created as a response to the unsuccessful use of online ad networks, which resulted in huge advertising expenses with limited results. Trading desks offer more competitive pricing than the common ad network.