Graphic User Interface. It's a visual representation of all the zeros and ones that the computer uses to process your commands. Xerox first touched on it with routementary buttons and commands. Then Apple, then Microsoft, and so on.
Nowadays its referred to as UI and it's married to UX. A User Interface (UI) creates the User Experience (UX).Like the dashboard of your car, the button format on your microwave, to your game's loading screen to your mobile phone, down to the websites you use. You're interacting with a device. And that graphic user interface, or UI, is helping you do that.
It's become so important that businesses are setting aside money, a lot of money in fact, just to ensure that they have an industry expert supporting their needs for their users' experiences. And if you ever wonder why a user's experience is so important, or why websites are changing their user experience to look more like mobile devices...watch a five year old use an iPhone or iPad.