Unicast packets are sent from one single host to another single host.
Broadcast packets have the host (and/or subnet) portion of the address set to all hosts on the local subnet. Most modern routers will block IP broadcast traffic and restrict it to the local subnet, which is called a broadcast domain.
Multicast packets are sent from a single device to a specific set of hosts, no matter what the IP and subnet combination is.
ICND1 and ICND2 break down