Hi
Unicast/Multicast is a network (IPv4) concept. Briefly, Unicast is delivering a message to one node, whereas multicast is delivering a message to many nodes. In both cases you send only once. Incidentally, IPv6 introduces the concept of anycast.
In
java you can see this in use in RMI in the remote reference layer.
This is very brief, but hopefully helps point you in the right direction