The word "terminal" comes from early computer systems that were used to send commands to other computers. Terminals often consist of just a keyboard and monitor, with a connection to another computer. The purpose of a terminal is not to process information (like a typical computer), but to send commands to another system. For example, a network administrator may use a terminal to log in to a network and manage devices connected to the network.
With modern computers, the word "terminal" usually refers to a terminal program, or emulator, which provides a text-based interface for typing commands. This type of program is often abbreviated "TTY" and may also be referred to as a command-line interface. Terminal programs are available for all major computing platforms and are typically included with the operating system. For example, Windows includes the program "cmd.exe," which is a command-line interface that can be used to run DOS commands and to connect to other servers. Mac OS X includes a program appropriately called "Terminal," which can be used to run Unix commands within the Mac OS or to access other machines. Unix typically includes a program called "xterm," which can run BASH or other Unix shells.
Unlike most modern computer programs, terminal emulators do not have very user-friendly interfaces. In order to use a terminal program effectively, you need to know Unix (or DOS) commands. Typing "help" or "man" may provide you with an overview of commands, though it can be difficult to use them if you don't know what parameters to include with each command. This Unix Commands Reference may be helpful if you are using a Unix terminal for the first time.