The “UI” in UI design stands for “user interface.” The user interface is the graphical layout of an application. It consists of the buttons users click on, the text they read, the images, sliders, text entry fields, and all the rest of the items the user interacts with. This includes screen layout, transitions, interface animations and every single micro-interaction.
“UX” stands for “user experience.” A user’s experience of the app is determined by how they interact with it. User experience is determined by how easy or difficult it is to interact with the user interface elements that the UI designers have created.