UX design or user experience design, is the process by which a pain point or need is identified. Then a rough prototype is drawn which is later validated (or invalidated) through testing. When both the business model and the value proposition have been validated the product is built.
You can think of UI design or user interface design like this:
User Interface Design = Visual Design + Interaction Design.
Visual design is the look and feel of the site, the personality if you will; the brand. Interaction design is the way people interact with your site. When someone clicks a button on your site, does the button change in a noticeable way so they know they’ve been successful?
UX Makes Interfaces Useful
UI Makes Interfaces Beautiful
The research process of a UX designer involves doing a competitive analysis, developing personas, and then developing a minimum viable product or more apropos, minimum valuable product; a product that will be valuable to your targeted customer niche. This is validated through testing throughout the life-cycle of the product.
Once the user flows and wireframes are prototyped and tested, it’s the UI designer’s role to make them aesthetically pleasing. This includes choosing a color scheme and typography that will be both attractive and simple to use. However, color choices, typography and interactions are not based on the designer’s personal preference but rather on clearly articulated reasons specific to the personas developed by the UX designers. With these, UI designers implement a visual hierarchy that will serve as a guide to users, letting them know what to do and when to do it, in order to meet their objectives.