Multicasting is the ability to transmit a single stream to multiple subscribers at the same time. Unlike conventional streaming, it does not need one stream per recipient. Instead, there is one stream on any one segment of the network on which there is a subscriber. It is the task of the routers to track subscriptions and to create copies only on an as-needed basis. Unlike broadcasting, segments on which there are no subscribers do not receive the stream. Read the article at FreshMeat.