UI is the visible and interactive layer people use: buttons, menus, forms, controls, typography and other interface elements.
The interface is how a person operates the digital product in front of them.
In plain English
This belongs to the machinery or experience of a website. You do not need to know how to code it. You need to understand what changes for the visitor, the editor or the person maintaining the site: clarity, speed, accessibility, stability or how easy the site is to change later.
A real example
A checkout has fields, labels, buttons, error messages and payment choices. Those pieces are UI. Whether the checkout feels easy and reassuring is part of UX.
What matters is not whether the technology sounds advanced, but whether the result is felt as a clearer, faster, safer or easier website.
Why it matters
Clear interface design reduces mistakes and makes actions obvious.
Technical choices eventually become human consequences. A setting that sounds obscure can decide whether a page loads, whether someone using a screen reader can use it, or whether a future redesign becomes simple or painful. That is the level at which the term becomes useful.
When do you actually need to care?
You need to care when the site is slow, difficult to use, hard to edit, inaccessible, unstable, expensive to maintain or becoming dependent on one fragile technical choice. You also need to care before migrations, redesigns and new integrations, because small technical decisions can become large constraints later.
Useful questions to ask
- What will a visitor or editor notice if this is done well?
- Who is responsible for maintaining it after launch?
- What happens if the technology stops working or needs to be replaced?
What should you do with this?
Ask what changes for the visitor, who is responsible for maintaining it, and what happens if it fails. If nobody can answer those three questions, the implementation is probably more complicated than it needs to be.
What this does not mean
UI and UX overlap, but they are not synonyms. UI is part of the experience.
The short version
UI is what people interact with. UX is what the whole interaction feels like and enables.
Related: UX and responsive design. You can also read about website strategy.
Further reading: W3C tips for designing accessible interfaces.