UI (user interface) refers to how a user interacts with an app. UX (user experience) is about all of it — from idea to execution. Both are integral to creating a successful mobile app, but for many people, UI and UX are hard to separate and think about separately. For example, some will say that good UX makes a UI easy to use or pleasurable; others will say that good UI makes a UX easy to use or pleasurable.
