All visual components in Android descend from the View class and are referred to generically as Views

The ViewGroup class is an extension of View designed to contain multiple Views. View Groups are used most commonly to manage the layout of child Views, but they can also be used to create atomic reusable components. View Groups that perform the former function are generally referred to as layouts

Assigning User Interfaces to Activities

A new Activity starts with a temptingly empty screen onto which you place your UI. To do so, call setContentView, passing in the View instance, or layout resource, to display. Because empty screens aren’t particularly inspiring, you will almost always use setContentView to assign an Activity’s UI when overriding its onCreate handler.

The setContentView method accepts either a layout’s resource ID or a single View instance. This lets you define your UI either in code or using the preferred technique of external layout resources

@Override
public void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.main);
}

Using layout resources decouples your presentation layer from the application logic, providing the flexibility to change the presentation without changing code. This makes it possible to specify different layouts optimized for different hardware configurations, even changing them at run time based on hardware changes (such as screen orientation changes).

You can obtain a reference to each of the Views within a layout using the findViewById method:

TextView myTextView = (TextView)findViewById(R.id.myTextView);