A Flutter application is made of widgets, and widgets are the description of a part of the user interface. Every user interaction, and everything that the user sees when navigating your app, is made of widgets. The app itself is a widget! 

That's why when you begin using Flutter, one of the concepts that you'll hear most often is that "in Flutter almost everything is a Widget." This is mostly true.

You use Dart to write widgets. If you have some experience in mobile or web programming, then you may find this a bit unsettling. Most of the other mobile frameworks use some form of XML or HTML to describe the user interface, and a full programming language for business logic. In Flutter, you use Dart to describe both—the user interface, as well as the business logic of your app. 

The app we'll build in this chapter is a single-screen app, with some text, a picture, and a button that, when clicked, gives the user a message. So, even if the app is extremely simple, you'll get to see many features of Flutter, including the use of widgets, styling text, downloading images from the web, and the creation of alerts.