An aspect of layout—like padding or alignment

The following example shows the “Hello world!” app using widgets from the
Material library. In this example, the widget tree is nested inside the
MaterialApp root widget.

The following images show “Hello world!” built from Material Design widgets.
You get more functionality for free than in the basic “Hello world!” app.

When writing an app, you’ll use two types of widgets:
StatelessWidget or StatefulWidget.
A StatelessWidget is just what it sounds like—a
widget with no state. A StatelessWidget is created once,
and never changes its appearance.
A StatefulWidget dynamically changes state based on data
received, or user input.

The important difference between stateless and stateful
widgets is that StatefulWidgets have a State object
that stores state data and carries it over
across tree rebuilds, so it’s not lost.