UX design refers to user experience design, while UI design stands for user interface design. Both of these are crucial to an IT product and need to work closely together.
Despite being very integral to each other, the roles themselves are quite different, involving distinct processes.
UX is User Experience
User experience (UX) design is the process of creating products that provide meaningful and relevant experiences to users. This involves the design of the entire process of acquiring and integrating the product, including aspects of branding, design, usability, and function.
UI is User Interface
User interface (UI) design is the process of making interfaces in software or computerized devices with a focus on looks or style. Designers aim to create designs users will find easy to use and pleasurable. UI design typically refers to graphical user interfaces but also includes others, such as voice-controlled ones.