Multicast - (1) "Multicasting" refers to sending the same message or content to a select group whereas "broadcasting" refers to sending the same message or content to everyone connected to a network, regardless of who wants to see or hear it.  An email related example of multicasting is the e-mail "mailing list" (e-groups) where people sign up to receive e-mail from everybody in the group (versus someone sending the same email to everybody on the Internet.)

To receive a multicast, a client must have access to a multicast-enabled network. In contrast, an unicast is a one-to-one connection in which one client receives a distinct stream from a server. 
(2) Broadcasting TV programming via the Internet using broadband.
http://en.wikipedia.org/wiki/Multicast
http://www.pcmag.com/encyclopedia_term/0


Multiplexer

Multicast-enabled network - A network that has routers that can interpret Class D IP addresses.