Flutter is an open-source UI software development toolkit created by Google. It is used to develop applications for Android, iOS, Linux, Mac, Windows, Google Fuchsia, and the web from a single codebase. Flutter is known for its fast development cycles, expressive and flexible UI, and native performance.

1. Single Codebase: Write once, run anywhere. Flutter allows developers to write a single codebase that can be compiled to run on multiple platforms.
2. Hot Reload: Flutter’s hot reload feature helps you quickly and easily experiment, build UIs, add features, and fix bugs. It allows you to see the results of your changes almost instantly.
3. Expressive and Flexible UI: Flutter provides a rich set of customizable widgets that help you build native interfaces in minutes.
4. Native Performance: Flutter’s widgets incorporate all critical platform differences such as scrolling, navigation, icons, and fonts to provide full native performance on both iOS and Android.
5. Strong Community and Ecosystem: Flutter has a growing community and a rich ecosystem of packages and plugins that extend its capabilities.

Flutter’s architecture is designed to be layered and extensible. Here’s a high-level overview:

1. Framework Layer: This is the topmost layer where the Flutter framework resides. It includes the Dart-based UI toolkit, which provides a rich set of pre-designed widgets.
2. Engine Layer: The engine is written in C++ and provides low-level rendering support using Google’s Skia graphics library. It also interfaces with platform-specific SDKs.
3. Embedder Layer: This layer is platform-specific and provides the necessary glue code to integrate the Flutter engine with the host operating system.

1. Fast Development: With features like hot reload, Flutter allows for a highly productive development experience.
2. Beautiful UIs: Flutter’s rich set of widgets and design capabilities enable developers to create visually appealing applications.
3. High Performance: Flutter applications are compiled directly to native ARM code, ensuring high performance on both iOS and Android.
4. Strong Community Support: The growing community and extensive documentation make it easier to find solutions and get help when needed.

Let’s create a simple "Hello World" application in Flutter to get a hands-on understanding.

1. Create a New Flutter Project: Open your terminal and run the following command:

   flutter create hello_world
   

   Navigate to the project directory:

   cd hello_world
   

2. Open the Project in Your IDE: Open the hello_world project in your preferred IDE (e.g., Visual Studio Code, Android Studio).

3. Modify the Main Dart File: Open the lib/main.dart file and replace its content with the following code:

   import 'package:flutter/material.dart';
   
   void main() {
     runApp(MyApp());
   }
   
   class MyApp extends StatelessWidget {
     @override
     Widget build(BuildContext context) {
       return MaterialApp(
         home: Scaffold(
           appBar: AppBar(
             title: Text('Hello World App'),
           ),
           body: Center(
             child: Text('Hello, World!'),
           ),
         ),
       );
     }
   }
   

4. Run the Application: Use the following command to run the application:

   flutter run
   

• import 'package:flutter/material.dart';: This line imports the Flutter material design library.
• void main() { runApp(MyApp()); }: The main function is the entry point of the application. runApp takes a widget and makes it the root of the widget tree.
• MyApp: This is a stateless widget that represents the application. It returns a MaterialApp widget, which is the top-level widget for a Flutter application.
• Scaffold: This widget provides a basic material design visual layout structure.
• AppBar: This widget creates a material design app bar.
• Center: This widget centers its child within itself.
• Text: This widget displays a string of text.

In this section, we introduced Flutter, its key features, and its architecture. We also walked through creating a simple "Hello World" application to give you a practical understanding of how Flutter works. In the next section, we will cover setting up the development environment to get you ready for more advanced Flutter development.