UI stands for “user interface design” and UX stands for “user experience design”. To put it more broadly, UI typically refers to the “look” of an application while UX refers to the “feel”. That is, UI constitutes the visual aesthetics of the app, while UX involves the functional ease of using the app.
UI typically includes visual design, such as color scheme, imagery, font choice and size, etc. In short, it encompasses all elements of the app’s interface.
UX, on the other hand, is somewhat more nebulous. UX encompasses slightly more subjective elements such as the size of buttons and text inputs, the placement of links, the flow of the app from interaction to interaction, and so on.