UI Design is what we commonly call web designer. The UI Designer’s role is to create high fidelity mock ups based on the wireframes a UX Designer might create. The designed mock ups are basically what the end user sees. The UI Designer will figure out interface design color schemes, button shapes, photography, typography and all other design elements in the system.
Typically the best interfaces are basically invisible to the user. I personally like to design clean and simple interfaces that don’t use unnecessary heavy and bloated elements. The key is to create consistent UI elements throughout the system, so users can start understanding the system through the design.