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. Any sort of visual element, interaction, or animation must all be designed.
“UX” stands for “user experience.” A user’s experience of the app is determined by how they interact with it. Is the experience smooth and intuitive or clunky and confusing? Does navigating the app feel logical or does it feel arbitrary? Does interacting with the app give people the sense that they’re efficiently accomplishing the tasks they set out to achieve or does it feel like a struggle? User experience is determined by how easy or difficult it is to interact with the user interface elements that the UI designers have created.