If you're new to the topic of terminal emulation, here is some of the most common jargon used in the industry.

Big Iron
(See Mainframe). Big Iron is a slang term for a mainframe; it refers to the bulky mainframes developed by IBM in the '60s and '70s.
Block Mode
A method of communication between a terminal and a mainframe or host system. User input is entered into the terminal and sent to the host when a form or field is completed and submitted; in 'blocks'. Some validation occurs on the terminal.
Character Mode
A method of communication between a terminal and a mainframe or host system. Each keystroke (character) of input is sent from the terminal to the host when input. Validation is handled by the host system.
Centralized computing
Computing done from a central location, using terminals or clients attached to the main computer. A mainframe is a perfect example of centralized computing.
Distributed computing
A system containing multiple computers that communicate over a network. It stands in opposition to centralized computing.
Graphical user interface
A type of interface allowing users to interact with a computer through images, rather than text based commands. Microsoft Windows is a well-known example of a graphical user interface -- or “GUI” for short.
A large, high-speed computer that usually supports many terminals. Mainframes are used mostly by large organizations for bulk data processing.
A smaller, lower cost version alternative to a mainframe which offers lesser power but a more affordable entry cost. Wang, DEC and Data General systems were all well known producers of these midrange systems along with IBM's highly successful AS/400 range...
Remote access
The ability to communicate with a computer from a location separate to where that computer is located.
SSH or Secure Shell.
A communications protocol that allows secure transmission of data. For host connection, SSH has largely replaced telnet due to the unencrypted nature of the latter. A quality terminal emulator should support SSH.
A computer program that serves the requests of other computers on a network. These computers are known as "clients". The term "server" and "host" are used interchangeably.
A proprietary communications protocol developed by IBM. Replaced by TCP/IP. Terminals such as the 3270 and 5250 originally made use of this protocol.
A device at which a user enters commands for a computer system. The terminal then displays the computer's return output. Sometimes used interchangeably with the term "Dumb Terminal", since it possesses either no or extremely limited computing power of its own; it is merely a means of accessing a host system...
Terminal emulator
A software program that mimics the role of a hardware terminal. Terminal emulation is used as a means of communicating with mainframes in the modern computing environment. Common terminal emulations include 3270 emulation, 5250 emulation and VT220 emulation.
A multi-user operating system commonly used by modern servers, and popular for its portability, stability and multitasking capabilities. Unix based operating systems largely replaced proprietary operating systems offered by manufacturers on early mainframe and minicomputer systems.
A hugely successful terminal product by Digital Equipment Corporation. Many terminals are variations on the VT100 standard.
A popular family of terminals produced by IBM. Often used in when connecting to an AS/400 minicomputer.
A widely used family of terminals produced by IBM for it's larger systems.