Pros and Cons of Ad Exchanges
An ad exchange is a marketplace in which publishers and advertisers can participate in an auction-based system for buying and selling online display advertising from a large group of participating sites. Publishers place their unsold inventory on the exchange, and buyers place bids to purchase the leftover inventory through an easy-to-use interface (kind of like paid search).
This auction system creates a competitive environment in which each bidder has equal access to the media, so relationships and budget sizes typically have no bearing. Ad exchanges are generally open to all ad agencies, advertisers, publishers, and ad networks. (Although publishers do have the right to reject ads they don’t want to run on their sites.)
Read more at http://www.clickz.com/showPage.html?page=3626493