So what exactly do UI and GUI stand for, and what's the difference? UI stands for user interface, and GUI (pronounced "gooey") stands for graphical user interface. To interface means to interact with, so the user interface is the set of devices that let the player interact with the game. The mouse, keyboard, game controller, touch screen, and so on are all part of the user interface. Graphical user interface is the subset of the user interface that is represented by graphics. So, onscreen buttons, drop-down menus, and icons are all part of a game's GUI.
This book will focus primarily on GUI design, but it will discuss some non-graphical aspects of user interface controls, like accessing data from the mouse, screen tap, keyboard, or controller.
As GUI is a subset of UI, many people (myself included) tend to just refer to the GUI as UI. In fact, Unity also refers to all the GUI items they provide templates for as UI. "Back in the day", the days before Unity 4.6, to implement visual UI in Unity, you had to use the "legacy GUI controls". These controls used the OnGUI() method. I believe one of the reasons that Unity refers to their new graphical user interface controls as UI rather than GUI is to make a distinction between the old GUI and the new UI. This, and the fact that I feel silly saying "gooey", is also why I refer to the graphical user interface we can implement in Unity as "UI".
Anything Unity replaces with new functionality gets marked as "legacy". Legacy items are only left available within Unity for backward compatibility.
As a general rule, if you are trying to find any information about working in Unity's UI and find reference to OnGUI(), move along! This code has been deprecated for a reason, and Unity no longer supports it. In fact, Unity explicitly states that you should not use it for a new project. There are a few diehard OnGUI() fans out there who want to hold on to the past, but I couldn't tell you why. In my experience, I have not found anything to be "undoable" in the "new" (it's not that new anymore) UI. While some things you may want to do will not be explicitly provided by the "new" UI system, a little bit of your own code can go a long way.
