TCP

The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. The protocol guarantees reliable and in-order delivery of data from sender to receiver. TCP also distinguishes data for multiple connections by concurrent applications (e.g., Web server and e-mail server) running on the same host.

TCP supports many of the Internet's most popular application protocols and resulting applications, including the World Wide Web, e-mail, File Transfer Protocol and Secure Shell.

UDP

(User Datagram Protocol) The UDP protocol is a protocol within the TCP/IP protocol suite used in place of TCP when a reliable delivery is not required. There is less processing of UDP packets than there is for TCP pakcets. UDP is mainly used for streaming audio and video, voice over IP (VoIP) and videoconferencing, because there is no time to retransmit erroneous or dropped packets.

If UDP is used and a reliable delivery is required, packet sequence checking and error notification must be written into the applications.

UDP is "connectionless" and does not use a handshake to start a session. It just sends out packets.