UX Design refers to the term User Experience Design, while UI Design stands for User Interface Design. Both elements are crucial to a product and work closely together. But despite their professional relationship, the roles themselves are quite different, referring to very different parts of the process and the design discipline. Where UX Design is a more analytical and technical field, UI Design is closer to what we refer to as graphic design, though the responsibilities are somewhat more complex.
There is a quaint little analogy I like to use in describing the different parts of a digital product:
If you imagine a digital product as the human body, the bones represent the code which give it structure. The organs represent the UX design: measuring and optimizing input and output for supporting life functions. And UI design represents the cosmetics of the body–its presentation, its senses and reactions.