Short for character user interface or command-line user interface, CUI is a way for users to interact with computer programs. It works by allowing the user (client) to issue commands as one or more lines of text (referred to as command lines) to a program. Good examples CUIs are MS-DOS and the Windows Command Prompt. One of the CUI's uses is that it provides an easy way to implement programming scripts.
Other examples of command lines
The command-line user interface was the primary method of communicating with a computer from the first machines, all the way through the 1980s. Although it may still be accessed in today's operating systems, it is utilized far less due to the ease of use and familiarity of the GUI (graphical user interface). The CUI, however, is still preferred by many advanced end users as its features provide them with more comprehensive control over an operating system's functions.