I found this article interesting and explicative about what UI and UX are.
Here is a short part of „the UX Umbrella“ that I found interesting:
The items that are sheltered by the umbrella have two purposeful omissions – user experience design and interface design.
User experience design is omitted because it is the loose term that encompasses all of the various disciplines. You’re never really doing any “user experience design” that isn’t just a combination of one or more of the things under the umbrella.
User interface design is omitted because it is the crossover between visual design (look and feel) and the interaction design (how the look and feel work). Combine those two and you have an interface. The interface is the result of the “solution design” that came before it.
A skillful interface designer understands the importance of providing the user with a solution to a defined problem.“