IP Multicast Introduction   (link)

http://ntrg.cs.tcd.ie/undergrad/4ba2/multicast/antony/
programmer-oriented multicasting introduction
This tutorial assumes basic familiarity with the socket programming abstraction found in many variants of the UNIX Operating System. This tutorial will illustrate how to use sockets to join an IP multicast group and send and receive data from multicast groups.

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ipmulti.htm
Internet Protocol (IP) multicast is a bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to thousands of corporate recipients and homes. Applications that take advantage of multicast include videoconferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news.