A ContentDistributionNetwork (CDN) is content "coming soon to a server near you!" - basically, it's an overlay network service that keeps up-to-date copies of data in a nearby cache or proxy node. This is useful for scalability and robustness, since it avoids a central point of failure and it distributes the bandwidth burdens.

http://en.wikipedia.org/wiki/Content_distribution_network

See also CoralWebPrx, ContentCentricNetworking, ReactiveDemandProgramming