Multicasting

Multicasting is a set of addressing methods, protocols, and network design principles that let a single source send, efficiently, to a group of recipients. The group may be predefined as made up of "well-known" addresses, or it may be dynamic, such as when participants join and leave a conference.

Broadcasting is a special case of multicast, where all possible destinations, perhaps within a certain area, are part of the destination group. Groucho Marx gave a good description of a broadcast environment when he said he wouldn't join any club that would have him as a member.