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Multicast packets are addressed to a "group" address, rather than an individual host address. On shared media networks (like 10Base2, 10Base5 or 10BastT [on a hub] Ethernet), sender only needs to send one packet which all (originally all in same L2 domain) hosts would see, and each host would determine, by looking at group address, whether it wanted to open the packet to "see" packet's contents.
Multicast on switches might be flooded (replicated) to all ports within the same L2 domain (pretty much like broadcasts). (This mimicked shared media behavior.)
"Modern" managed switches often support IGMP snooping where the switch will not flood (replicate) multicast packets to a port unless it "knows" a host on that port desires them.
By default, multicast, like broadcasts, isn't forwarded at L3.
Multicast can be multicast routed (i.e. L3 forwarded) across different L2 domains. Multicast routing might use a dedicated multicast routing protocol, such as DVMRP (basically RIP for multicast) or piggy back on a unicast routing protocol (i.e. PIM variants).
Multicast routing is "backwards" from unicast routing, as in rather than figuring how to deliver a packet to a specific host, multicast routing concerns itself with getting from receiver to sender.
