Unicast is a type of communication where data is sent from one computer to another computer, and in this type of communication, there is only one sender, and one receiver.
1) Browsing a website. (Webserver is the sender and your computer is the receiver.)
2) Downloading a file from a FTP Server. (FTP Server is the sender and your computer is the receiver.)
Multicast is a type of communication where multicast traffic addressed for a group of devices on the network. IP multicast traffic are sent to a group and only members of that group receive and/or process the Multicast traffic.
Devices which are interested in a particular Multicast traffic must join to that Multicast group to receive the traffic. IP Multicast Groups are identified by Multicast IP Addresses (IPv4 Class D Addresses)
In Multicast, the sender transmit only one copy of data and it is delivered and/or processed to many devices (Not as delivered and processed by all devices as in Broadcast) who are interested in that traffic.
Eg : Multicast Windows Deployment Services (WDS) OS deployment traffic, IP TV etc
Broadcast is a type of communication where data is sent from one computer once and a copy of that data will be forwarded to all the devices.
In Broadcast, there is only one sender and the data is sent only once. But the Broadcast data is delivered to all connected devices.
Switches by design will forward the broadcast traffic and Routers by design will drop the broadcast traffic. In other words, Routers will not allow a broadcast from one LAN to cross the Router and reach another Network Segment. The primary function of a Router is to divide a big Broadcast domain to Multiple smaller Broadcast domain.
Example: ARP Request message, DHCP DISCOVER Message