UX and UI often get used interchangeablyv, but they are two different things. Both have to do with your end user, or customer.
UX is the user experience. UI is the user interface. Let's break it down.
What is UX?
Jakob Nielson summarizes the user experience as "all aspects of the end-user's interaction with the company, its services, and its products."
A big-picture-view of excellent user experience goes beyond just one aspect of your website. Every discipline and department of your company merges to form the user experience, including product development, customer service, marketing, graphic design, and interface design.
Sometimes UX and UI get boiled down to a simple definition of usability. Nielsen says that although usability is an important aspect of user experience, it is by definition just one quality attribute of user interface.
Which brings us to our next question, what is UI?
User interface has various meanings, the most broad being the "space where humans and machine interaction occur." Sounds pretty cool, like science fiction.
Translated to a website design context, user interface means the space on a website that a user interacts with. This includes graphic design, navigation, forms, and all of the visual elements that a user needs to make decisions, find what they need, and take action on your website.
A good user interface makes browsing a website easy, effective, enjoyable, and produces the desired result for both the user and the owner of the website.
Responsive design is one way that developers and designers have addressed UI issues with multiple display sizes across mobile and desktop screens.
Nielsen gives an example of how good UI can still lead to a bad UX.
"As an example, consider a website with movie reviews. Even if the UI for finding a film is perfect, the UX will be poor for a user who wants information about a small independent release if the underlying database only contains movies from the major studios."
So, when you think of UX and UI, remember they are related but separate concepts. One is the larger view of everything that goes into your customer's experience with your business, services, and products (UX). The other is the more specific, technical details of how the user interacts with your website to effectively learn more, buy or share with others (UI).