The terms UI and UX get floated around a lot on the internet, usually together and often interchangeably, but—as those of us who have gotten them backwards can attest—there’s a big difference. If you’re curious about understanding the difference and/or just want to stop annoying the company designer by getting their job wrong, you’ve come to the right place.
The Short Version
UI is what the user uses to interact with an application, and UX is how they interact with an application.
Wait, really?
Yeah, pretty much.
You can’t just end the article there; I have expectations. I came here for Venn Diagrams and I won’t leave until you show me them.
Okay, fine.
In a sense, UX design has existed since the dawn of civilization. The First UX Designer was probably somebody in Ancient Sumer who figured out you could add a little handle onto a jug to make it easier to carry, or maybe even whoever invented the wheel—a huge leap from the previous hardware Rock, which was good for throwing but lacked key features that allowed it to roll effectively.
UI design, comparatively, came about much later. There’s a lot of argument, but most people put it somewhere between 1970–1990, with the rise of the personal computer. Previous information hardware (like, for example, the book) was relatively simple for the layman to use—put hand on page, turn page, try to avoid getting a papercut. Computers, on the other hand, are incredibly complex, and few people have the training to operate them without some sort of interface that translates the underlying code into comprehensible inputs and makes it easier to manipulative for end-users.
Or, to break it down further:
UX design is about identifying problems, figuring out how people actually use the thing and building it so it’s better-suited for their actual needs, and UI design is about making software interfaces that take complex code and represent it in a way users can actually use.
Wait, does that mean … sometimes they’re the same thing? 
Yeah so, that’s where it gets more complicated. In the 80s and 90s, that Venn Diagram was a circle—one person would do both jobs, and it was never really an issue. You’ll still see them listed together as UX/UI design and for good reason—they’re two branches of the same tree. However, as our applications become more powerful, complex, and commonplace, they’ve split apart and now you’re more likely to have a UX team and a UI team working together in concert.
NOTE: UX Design is broader than software, but for the purposes of this article going forward, we’re specifically referring to Software UX Design.
So what does a UX Designer actually do?
Understands, investigates, and puzzles out how people are actually interacting with an application. This usually involves a mixture of well-chosen datasets and human psychology. UX Designers often use something called a User Persona and/or a User Story, which is like a target demographic except it’s just one person. A user persona is a hypothetical user created for testing and design purposes, and the designer attempts to get inside their head and navigate the application as they would.
User personas are often composites of data based on real people. People are, after all, quirky and weird and often do things in unexpected ways; basing your persona off real data tends to give them a finesse that’s hard to get elsewhere.
UX design is often based around pathways: using analytics to follow users moving through a particular site to understand how they’re navigating the space and how to modify that space to make things run more smoothly.
It’s also the job of the UX designer to factor in things like accessibility for people with (for example) vision impairment or mobility issues in their hands. A smaller button might be fine for your office testers, but for somebody with arthritis it might present a real problem—that’s 54 million people in the US alone (23% of the entire population!) who will have difficulty using your app.
And that’s sort of the crux of it: modern UX design is about understanding how people think and act, then creating design patterns to make things work better for them. What about UI designers?
Here’s the bit where I talk about what UI Designers actually do
While UX comes at it from a direction of psychology, UI comes at it from a graphic design and frontend development standpoint. The UX designer figures out that people with arthritis might have issues with that small button, then the UI designer goes and makes a better button.
The UX designer analysed the data, figured out how people were moving through the site, and designed better pathing; the UI designer goes out and actually beats the trail. Architect vs Engineer. You could have the same person doing both, but splitting the tasks up allows your designer to focus deeper on being the best at their one particular skillset.
What does good UI/UX look like?
Usually, nothing. Good UI and UX are invisible: we only tend to notice them when something has gone wrong. There are some exceptional designers who play with that and have more obtrusive UIs that have their own distinct appeal, but the rule of thumb is that the less the user notices it, the better it is.
That seems too easy, doesn’t it?
Oh yeah, definitely. There’s a thousand different types of design all overlapping in different ways and we’ve only really sailed by the tip of the iceberg. My objective here was to help disambiguate two commonly-conflated terms, not to write a thesis on which department to put the sound designer in. You absolutely could write that thesis, though; we’re sailing on top of some deep and strange waters. If you’re curious, I invite you to dive deeper, because there’s a lot of fascinating stuff to explore. UI/UX can involve information architecture, font rendering and typography, even copywriting. Font rendering, and how web fonts are changing the game
C’mon, one last Venn Diagram. Do it for me.
Okay you scamp, just the one. 
In Summary: UI and UX design often overlap in critical ways, but at the end of the day, UI is the what and UX is the how and why.
And that’s that.
