What is the difference between a StatelessWidget and a StatefulWidget in Flutter, and how do you actually properly decide which specific one to genuinely use for a given particular widget?

A StatelessWidget describes a genuine part of your interface that never actually needs to change its own internal appearance once it has genuinely already been built, meaning it has no genuinely mutable internal state of its own whatsoever, while a StatefulWidget can properly hold and correctly manage its own genuinely mutable internal state that can meaningfully change over time in direct response to user interaction or other events, and choosing correctly between the two genuinely comes down to a simple key question, specifically whether that particular widget actually needs to remember and properly update some piece of its own internal data over time, such as whether a checkbox is currently checked, or whether it can instead simply always be fully and completely determined just directly from its own externally received input parameters alone.

class Greeting extends StatelessWidget {
  final String name;
  const Greeting({required this.name});
  @override
  Widget build(BuildContext context) => Text('Hello, $name');
}