UDP: This is a communications transport protocol layer that is an alternative to the Transmission Control Protocol (TCP) layer. Like TCP, it interfaces with the Internet Protocol (IP) layer. UDP, however, does not provide the data gathering reliability of TCP. For example, it does not provide sequencing of the packets that the data arrives in. This means that the application program must be able to provide these services.

USB: Universal Serial Bus: a 12Mbps serial bus for PC peripherals designed for low and medium speed devices such as keyboards, monitors, tape drives, etc.

User: Any person who interacts directly with a computer system.

User ID: A unique character string that identifies users.

User Identification: User identification is the process by which a user identifies himself to the system as a valid user. (As opposed to authentication, which is the process of establishing that the user is indeed that user and has a right to use the system.)

User Interface: The part of an application that the user works with. User interfaces can be text-driven, such as DOS, or graphical, such as Windows.