The UI (user interface) is the graphical layout of an application. These interfaces should not only be functional, but they should be easy to use and visually appealing.
This includes screen layout, transitions, interface animations and every single micro-interaction. All of these visual elements or animations that allow you to interact with the app must be designed.
The UX (user experience) of the app is determined by how they interact with it.
User experience design is the process of planning the experience a person has when they interact with a product.
UX design focuses on the interaction that a human user has with everyday products and services.