UX refers to the term User Experience while UI stands for User Interface. But despite their vicinity, there are lot more to their execution process and design discipline.
UX design relates to the overall user interaction and experience with an application or a website. It is well known that great user experience results in return customers and great app usage. Hence the more importance. Typically, the UX Design process include research, design, prototyping, testing and measurement. Lot of factors influence UX process such as usability, utility, accessibility, design/aesthetics, performance, ergonomics, overall human interaction and marketing.
On the other hand, User Interface (UI) is more about visual design and information design around screens. Though a lot of process go into this graphical element, UI as the name implies, about the things the user will actually see and interact. It delivers tangible results and focuses on how an app or product surface look and functions.
Detailly discussed here https://goo.gl/xQUJn7