Experience and interface
User experience (UX) covers the whole experience of using a product, from knowing what to do to getting a useful result. User interface (UI) is the part people see and use: screens, buttons, forms, labels, and messages.
A checkout can look polished but feel frustrating if delivery costs appear only at the end or an error empties the basket. The interface is one part of UX; speed, trust, accessibility, and how the product handles errors matter too.
Design for the problem
Designers may research users’ needs, map steps, sketch screens, test wording, and work with developers on how the product responds. Teams use usability testing to see where people struggle. When someone asks for “a UX change”, clarify the problem before changing colours or layout.