The five types of "Casting"

Unicast, Broadcast, Multicast, Narrowcast, and Anycast

In order to understand IP addressing, the types of IP transmissions (casts) need to be defined.

Unicast - to transmit a single message to a single receiver. 

Multicast - to transmit a single message to a select group of recipients. A simple example of multicasting is sending an e-mail message to a mailing list. Teleconferencing and videoconferencing also use multicasting, and often use proprietary protocols in addition to the existing multicast standards (IP Multicast, Mbone, etc).  See RFC 1301     RFC 2932     and     RFC 3171

Narrowcast - to transmit a single message to a select group of recipients who have subscribed. Narrowcast is similar to Multicast and the two terms are often used interchangeably.  However, narrowcast refers to the business model whereas multicast refers to the actual technology used to transmit the data. Cable television is an example of narrowcasting since the cable TV signals are sent only to homes that have subscribed to the cable service.  Perhaps the best example of narrowcasting are e-mail lists where messages are sent only to individuals who subscribe to the list.

Broadcast - to transmit a single message to everyone connected to a network.  Note that multicasting refers to sending a message to a select group whereas broadcasting refers to sending a message to everyone connected to a network.

Anycast (IP v6) - to transmit a single message to the nearest of a group of receivers. Anycast is used in IPv6 as a method of updating routing tables, as in the 6to4 Anycast protocol. One host initiates an update of a router table for a group of hosts, sending the data to the nearest host. That host then sends the message on to its nearest router until all the routing tables in that group are updated.  See RFC 3056 and RFC 3068.