Shoutcast servers are radio servers.
A CDN is a content delivery network, which is "a system of computers containing copies of data, placed at various points in a network so as to maximize bandwidth for access to the data from clients throughout the network. A client accesses a copy of the data near to the client, as opposed to all clients accessing the same central server, so as to avoid bottlenecks near that server." (Wikipedia
Basically, static files are cached on varies servers spread out geographically. When a visitor needs that file, it will pull it from the closest server, increasing your loading speed.