When it comes to designing and laying out the screens of your application, you have two main options, writing code or using XAML. If you've ever done any WPF (Windows Presentation Foundation) or Silverlight development, then you're probably already familiar with XAML. XAML is the eXtensible Application Markup Language that was created to help define an application's look and feel without having to handle it all in code. Xamarin.Forms works with both options. It will ultimately be up to you to decide which option you prefer.