In Android, the user interface is defined in a layout. A layout can be declared in XML or created dynamically in code. (It's recommended to declare the layout in XML rather than in code to keep the presentation layer separate from the implementation layer.) A layout can define an individual ListItem, a fragment, or even the entire activity. Layout files are stored in the /res/layout folder and referenced in code with the following identifier: R.layout.<filename_without_extension>.
Android provides a useful variety of Layout classes that contain and organize individual elements of an activity (such as buttons, checkboxes, and other Views). The ViewGroup object is a container object that serves as the base class for Android's family of Layout classes. The Views placed in a layout form a hierarchy, with the topmost layout being the parent.
Android...
