The document discusses user interfaces (UIs). It defines a UI as the way a person interacts with electronic devices like computers, tablets, and smartphones. It includes screens, menus, icons, keyboard shortcuts, gestures, and voice commands. The document then provides examples of UIs like mice, remote controls, search engines, and ATMs. It outlines the evolution of UIs from early command line interfaces to modern touchscreen interfaces. The document concludes that UIs are changing rapidly and may become invisible in the future as new technologies like brain-computer interfaces emerge.