TCP/IP - Transmission Control Protocol/Internet Protocol. This is the standard protocol for data transmission over the Internet.
TCP - Transmission Control Protocol - TCP and UDP (User Datagram Protocol) are the two transport protocols in TCP/IP. TCP ensures that a message is sent accurately and in its entirety. However, for real-time voice and video, there is really no time or reason to correct errors, and UDP is used instead.
UDP - User Datagram Protocol - A protocol within the TCP/IP protocol suite that is used in place of TCP when a reliable delivery is not required. For example, UDP is used for real-time audio and video traffic where lost packets are simply ignored, because there is no time to retransmit. If UDP is used and a reliable delivery is required, packet sequence checking and error notification must be written into the applications.
VPN - Virtual Private Network that actually exists within a public network. This consists of a point-to-point tunnel through which users can send and receive data. The data packets are encrypted to provide for a true private connection to the endpoint (i.e. - corporate network). These packets cannot be decrypted without the correct encryption keys. Once the VPN tunnel is established, the client machine is authenticated and registered on the network. Given the proper privileges, it can then communicate directly with other machines as if it were actually on that local network.
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