A Graphical User Interface (GUI; pronounced "gooey") is an alternative to command-line tool use. The purpose of a GUI is to give users an intuitive, visual interface for interacting with the data. The only GUI provided in CIAO is ds9.
The ds9 GUI is used for both image display - including automatically binning up an event file - and for tabular display (which is what the prism GUI was used for prior).
The prism GUI was removed in CIAO 4.14. It provided the user with the ability to view (both numerically and graphically) the contents of data files. Most of the functionality can now be done in ds9.
The peg GUI was removed in CIAO 4.13. peg was a graphical parameter editor. It allowed you to set parameters for tools and run them without having to use command-line tools like pset and plist.
The taskmonitor GUI was removed in CIAO 4.13. It allowed users to run a task as a background process and display the output.
The ChIPS GUI - which enabled easy manipulation of plots created by ChIPS - was removed, along with the rest of ChIPS - in CIAO 4.12. The Matplotlib plotting package provides some graphical capabilities - such as zoom and pan - depending on what backend is in use.