CLI (Command Line Interface): It is basically a command-line program that usually ACCEPTS text as input to execute or run functions of the operating system. It allows users to type declarative commands simply to GIVE instructions to the computer to perform or execute operations. It usually requires LESS memory to use as compared to other interfaces as WELL as it does not require Windows and a low-resolution monitor can be used. It usually provides greater flexibility of use and can also be used to perform things easily that are most difficult to do with GUI.  

GUI (Graphical User Interface): It is basically a human-computer interface that allows users to interact with electronic devices through graphical icons and visual indicators. The use of these graphical elements or icons makes it easier for users to interact with the system. It is visually intuitive and allows higher productivity. It is usually a COMBINATION of graphical and textual interaction that uses menus, buttons, message boxes, etc.