User interface is not a part of UX either, it is in fact the result of UX. And by result, I mean the outcome of good UX.
In a world littered with abbreviations it is very easy to get carried away. To put two or three letter abbreviations together because they tend to belong to the same subject. As with ID (interaction design) UX, and UI. But, just because it fits, doesn’t make it right.
My clarification (open to criticism):
User experience is the art of trying to understand user needs and business goals and then merging them to create a near perfect world. The user’s experience is almost never only in an application, service or solutions, it is more often all around them as well. If we focus solely on the experience in the application, we will miss crucial surrounding information that almost always affects the user’s behavior and needs. Needs that can be based on time, money, space, other systems or almost anything really.
User interface is the front of an application, service, or solutions facing the user. The user interface is the point where the user meets the system and magic (supposedly) happens. The buttons, scrollbars, and text fields are elements of the user interface.
Interaction design is the art of deduction. This is where an interaction designer interprets user needs and business goals to attempt to produce blueprints of the best user interface possible.
So what is the point of this clarification?
It is important to know, that UX is not a misty world of user needs that can be tended to later on. It’s an important constant of any project that has users in mind. UX needs to be a part of the whole, from A-Z. Think of UX as the whisk to your batter; you will need it to mix in every single ingredient.
Thinking that UX is only producing a user interface is not a way to incorporate it as a whole – much like not mixing in your ingredients until the very end, you will end up with impossible lumps.
When UX is part of the entire project, then the outcome will be a user interface that benefits both the user and the business.
Good UX combined with a purpose, a goal, a pinch of design, and a splash of technical know-how will produce - a great UI.