Multicast is one of three ways you can use the IP protocol.
With multicast one computer sends traffic to a special IP address.[dead link] Other people can then choose to register to this IP address and then receive the traffic. Its advantage is that the server can send a single datastream, and then an endless number of clients can receive this data. This avoids having to send the same data on a single network connection. (The negotiation process to set up this connection is called the IGMP protocol.)
Multicast is not always supported. Your OS needs to know how to handle it (Windows, Mac OS X, and the Linux kernel do). And you need suitable network equipment (routers, switches, etc.). Also you can only do multicast on internet if your ISP is connected to the MBONE. Only a limited amount of end users can receive multicast from the internet; most of them would be universities and research centres.
However, most LANs can easily support Multicast. Beware that if your switch or routers don't support it, some types of network equipment actually broadcast the traffic. This might be a very undesirable behaviour.
See Also: Cisco detailed Multicast explanation