This page describes the Angular Material Design library and how it is used in ADF.
Google's Material Design is an example of a design language, a general set of principles and ideas that are used to give designs a consistent look and feel. A design language might be used in-house by a company to maintain a "family resemblance" between its products or to produce user-friendly signage. Material Design is Google's approach to keeping user interfaces consistent between apps, using insights from product design and cognitive psychology.
Angular Material is a set of components, styles and other GUI elements that follow the Material Design guidelines. ADF uses Angular Material to implement its components but you can also use it directly when creating your own components or modifying the ones provided by ADF.
Material Design doesn't enforce one single color scheme but it does specify a number of themes. A theme is a set of colors in various shades, each of which lends itself to a particular task in the GUI. For example, using the suggested shades for status bars, backgrounds and shadows helps to give the correct emphasis to these elements and ensure they look familiar to the user. The Material Design themes documentation has more information as well as color swatches and other resources.
An advantage of using themes is that one theme can easily be replaced by another - the CSS styling is designed so that a given class name in one theme plays an equivalent role in all other themes. See the Theming page for details of how to apply an off-the-shelf theme to your ADF app or to create your own theme.
Angular Material implements a variety of GUI components. These include controls like radio buttons and checkboxes but also structures for layout (lists, grids, etc) and navigation (toolbars, sidebars, menus). See the components section of the Angular Material docs for more information.
Material Design has extensive guidelines for the design of icons. These images serve as visual indicators for GUI functions and data elements (eg, a small graphic of a person could denote a user). A selection of standard icons is also available for common items. For example, a microphone icon indicates audio input while a magnifying glass emphasizes a search box. See the Material Design system icon docs for further information and to download the set of standard icon images.