UI (user interface) design is a subset of the overall UX (user experience) discipline. If your line of work involves either of these fields, hopefully you know this, but you'd be surprised how often the terms are confused and conflated by people who should know better.
UI is focused primarily on the interface between people and products or services. UI is more than visual or graphic design: it can involve other interaction methods such as voice and touch.
UX, on the other hand, is a broader discipline and includes, but is not limited to, activities like user research, content strategy, information architecture, usability evaluation, and — yes — UI.
Note: In the past I've said that while I'm a UX'er who also does UI, I tend to describe myself as a designer. When I use the terms UX and UI above, I'm referring specifically to disciplines or areas of expertise, not job titles.