A command line is a string given to a command interpreter that tells it actions to take, such as running programs or copying files. The interpreter processes command lines with escapes and substitutions.
A command-line interface (CLI) is a mechanism for interacting with a computer operating system or software by typing commands to perform specific tasks. This text-only interface contrasts with the use of a mouse pointer with a graphical user interface (GUI) to click on options, or menus on a text user interface (TUI) to select options.
This method of instructing a computer to perform a given task is referred to as entering a command. The system waits for the user to conclude the submitting of the text command by pressing the Enter key (a descendant of the carriage return key of a typewriter keyboard). A command-line interpreter then receives, parses, and executes the requested user command. The command-line interpreter may be run in a text terminal or in a terminal emulator window as a remote shell client such as PuTTY.
Upon completion, the command usually returns output to the user in the form of text lines on the CLI.
See also: Wikipedia Command-line_interface.