Graphical User Interface

A visual novel's Graphical User Interface, also known as its GUI (pronounced either “gee-you-eye” or “gooey”), is any screen that uses clickable images to let a player interact with the game. This includes things such as: the text box and option buttons (skip, save; etc), choice selections, and all menus (ex: main/title/start, settings, save, gallery, extras; etc).

Ideally, GUI is designed custom for the game to reflect its tone and theme. When in doubt, it is recommended you go simple and no-frills rather than detract from the game by being intrusive. Your GUI artist might not be the same artist you use for your sprites or CGs, as the skill set is different.

A good resource: http://visual-novel-interfaces.tumblr.com/