User interface and experience, almost always written as UI and UX, is the combined discipline of how a product looks and how it feels to use. UI is the visible surface of screens and controls. UX is the wider journey of getting something done, including the feelings, the friction and the small wins along the way.
A simple way to picture the pair: UI is the set and the lighting of a stage play, while UX is whether the story actually moves you. You can have a gorgeous set with a plot that drags, or a moving story performed on a bare stage with a single spotlight. The best results need both working together, which is why UI design and UX design are usually planned side by side rather than handed off in sequence from one team to the next.
In practice the two overlap constantly. The UX side maps the user journey, decides what goes on each screen and in what order, then the UI side gives that structure its colours, spacing, type and motion. When the split works, a user never notices the seam between them. The thing simply looks right and behaves the way they already expected it to, which is the highest praise a product can get.
At TopDevs we treat UI and UX as one job rather than two separate departments, so the products we ship are both pleasant to look at and genuinely easy to get through from the first click. A handsome screen that wastes your time is still a failure, and we would rather not ship one.
