UI in software is all about layout of text, images, navigation and controls on screen and today that screen can be anything from a watch face to a wide-screen TV. UI design incorporates the use of colours, font-sizes, typography, images and white space to create layouts that are visually appealing and user-friendly.
Psychology plays a role too, by understanding how people respond to software and in particular the layout of an interface it is possible to optimise their experience and influence their choices, indeed in certain contexts UI design in software becomes more about marketing than providing a good user experience.