GUI= Graphical User Interface , CLI = Command Line Interface

when a software is said have GUI , it simply means that you can operate the software using images/graphical icons. GUI Softwares are user-friendly.
An example of GUI software is Microsoft Windows XP where u can click icons

CLI on the other hand is operated by text or should i say with mainly the use of a keyboard ... An example of CLI software is Microsoft DOS ( MS DOS ).

CLICK START >> RUN and type " cmd " ( without the quotes ) to experience what CLI is like :)