Multicast is easier to explain over Unicast. Multicast is a broadcast UDP option for sending a packet/announcement over to a group that is listening on a specific multicast address and port over which the announcement is sent.There is a defined range for valid Multicast address (184.108.40.206 to 220.127.116.11). Everyone listening on the given address hears the announcement just like following a Twitter post. Some limitations with Multicast is the TTL (time to live) across machines/subnets/routers needs to be adjusted and the routers configured to re-transmit the multicast packet across subnets
Unicast is more of a point to point UDP option to send the packet to a specific member and not everyone.That way, unicast is more of a private conversation between two individuals while multicast is more of as hout to a group or room. Both are UDP based, so there can be losses unlike TCP that handles re-transmissions on message loss. But Unicast can span across routers and does not have to worry about TTL without the everyone hearing the announcement. So, Network Admins in general prefer to go with Unicast over Multicast for these reasons.