UX Design refers to the term User Experience Design, while UI Design stands for User Interface Design. They work closely together and are crucial to the success of a product.
"User experience design (UXD or UED) is the process of enhancing customer satisfaction and loyalty by improving the usability, ease of use, and pleasure provided in the interaction between the customer and the product." - Wikipedia
The UX role is complex, challenging, and multi-faceted involving part marketer, part designer, and part project manager.
What's involved in this process including strategy, content, wireframing, prototyping, executing, and analytics:
"User interface design (UI) or user interface engineering is the design of user interfaces for machines and software, such as computers, home appliances, mobile devices, and other electronic devices, with the focus on maximizing the user experience. The goal of user interface design is to make the user's interaction as simple and efficient as possible, in terms of accomplishing user goals (user-centered design)." - Wikipedia
The UI role is that of a visual and interactive designer and plays a key role in the creation of the digital interface and customer trust of a brand.
What's involved in this process including the look, feel, responsiveness, and interactivity: