Earlier I was so confused with these terms..may be many of others have same problems like me or may be not.
Here I tried to explain in simple way(Also copied many things from internet to make it simple and easy to understand. )
Unicast packets are sent from host to host. The communication is from a single host to another single host. There is one device transmitting a message destined for one receiver. In another words, Unicast transmission is between one-to-one nodes (involving two nodes only).
Examples of Unicast transmission are http, smtp, telnet, ssh, pop3 where the request for information is directed from one sender to only one receiver at the other end.
To understand this better, try to imagine a scenario where one computer’s request for the URL(google.com) is received by every other computer that’s connected to the Internet. This way Internet will be flooded with such requests and therefore there is a strong need for Unicast transmission on small and large networks. All Ethernet and IP networks support this type of transmission.
Unicast uses TCP for its communication and packet transmission.
Broadcast is when a single device is transmitting a message to all other devices in a given address range. This broadcast could reach all hosts on the subnet, all subnets, or all hosts on all subnets. Broadcast packets have the host (and/or subnet) portion of the address set to all ones. By design, most modern routers will block IP broadcast traffic and restrict it to the local subnet.
In other words, broadcast is a type of transmission in which information is sent from just one computer but is received by all the computers connected to the network. This would mean that every time a computer or a node would transmit a packet of type ‘broadcast’, all the other computers will receive that information packet.
Examples of broadcast are many but one that would be best to understand here is a computer booting up and requesting for a IP address. In this case, the computer which is booting up and requesting for an IP address does not know which computer will be able to provide it with an IP address. It will then broadcast a request packet on the network which will be received by all the other computers but will be acknowledged by only the computer acting as a DHCP server. One other example would be that of the ARP (Address Resolution Protocol) which will broadcast the address resolution request to all other computers on the network.
Broadcast uses TCP for its communication and packet transmission.
Multicast is a very much different from Unicast and Multicast in definition and application as well. It is a type of transmission or communication in which there may be more than one senders and the information sent is meant for a set of receivers. Please note that sometimes information might not be directed towards any receiver at all.
Multicast is a special protocol for use with IP. Multicast enables a single device to communicate with a specific set of hosts, not defined by any standard IP address and mask combination. This allows for communication that resembles a conference call. Anyone from anywhere can join the conference, and everyone at the conference hears what the speaker has to say. The speaker’s message isn’t broadcasted everywhere, but only to those in the conference call itself. A special set of addresses is used for multicast communication.
Multicast operates on a different class of IP address (Class D IP Series). All the other computers electing to receive information in a multicasted network must be a part of Multicast IP based network. Also, unlike Unicast and Broadcast which uses TCP for its communication, Multicast uses UDP. This is only because TCP does not support Multicast mode of communication.
Example of Multicast based network is video transmission network in which one computer needs to transmit video channel to a specific group of computers. This way when the other computers are also a part of this Multicast IP network, they will be able to receive same set of data at the same time. Multicast offers savings on bandwidth and is the preferred way of data communication when data is to transmitted to a set of computers.