User interface (UI) is the visual and interactive layer through which a user controls a product, distinct from the broader user experience the interface creates. UI includes layout, typography, color, controls (buttons, fields, menus), iconography, visual hierarchy, motion, and microinteractions. It is what the user actually sees and touches, while UX is everything they feel and accomplish.
The components of UI design cluster into several categories: layout and grid (how elements are arranged spatially), typography (typeface, weight, size, line height, the hierarchy of text), color (palette, contrast, semantic use of color for states like error/success), controls and components (buttons, inputs, dropdowns, toggles, sliders), ...