The goal of a User Interface Designer, often known as a UI Designer, is to design an interface which makes the user’s interaction as simple and efficient as possible.
Unlike UX designers who are concerned with the overall feel of the product, user interface designers are particular about how the product is laid out. They are in charge of designing each screen or page with which a user interacts and ensuring that the UI visually communicates the path that a UX designer has laid out.
UI designers are also typically responsible for creating a cohesive style guide and ensuring that a consistent design language is applied across the product. Maintaining consistency in visual elements and defining behaviour such as how to display error or warning states fall under the purview of a UI designer.