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TCP/IP and UDP/IP communications
There are two communication protocols that one can use for socket programming: datagram communication and stream communication.

Datagram communication:
The datagram communication protocol, known as UDP (user datagram protocol), is a connectionless protocol, meaning that each time you send datagrams, you also need to send the local socket descriptor and the receiving socket's address. As you can tell, additional data must be sent each time a communication is made.

Stream communication:
The stream communication protocol is known as TCP (transfer control protocol). Unlike UDP, TCP is a connection-oriented protocol. In order to do communication over the TCP protocol, a connection must first be established between the pair of sockets. While one of the sockets listens for a connection request (server), the other asks for a connection (client). Once two sockets have been connected, they can be used to transmit data in both (or either one of the) directions.[/ALIGN][/ALIGN][/ALIGN]
http://www.javaworld.com/jw-12-1996/jw-12-sockets.html?page=2