WPF

The fundamental of WPF programming is very familiar to the Windows and web Application. You can instantiate classes, set properties, call methods, and handle events, all using your favorite .NET Framework programming language, such as C# or Visual Basic. WPF exists as a subset of .NET Framework types that are for the most part located in the System.Windows namespace.

WPF includes additional programming constructs that enhance properties and events: Dependency Properties and Routed Events

WPF offers additional programming enhancements for Windows client application development. One obvious enhancement is the ability to develop an application using both markup and code-behind, an experience that ASP.NET developers should be familiar with. Development and maintenance costs are reduced because appearance-specific markup is not tightly coupled with behavior-specific code.

Development is more efficient because designers can implement an application's appearance simultaneously with developers who are implementing the application's behavior.
Multiple design tools can be used to implement and share XAML markup, to target the requirements of the application development contributors.
Globalization and localization for WPF applications is greatly simplified

Markup

XAML is an XML-based markup language that is used to implement an application's appearance declaratively. It is typically used to create windows, dialog boxes, pages, and user controls, and to fill them with controls, shapes, and graphics.

Code-Behind

The main behavior of an application is to implement the functionality that responds to user interactions, including handling events (for example, clicking a menu, tool bar, or button) and calling business logic and data access logic in response. In WPF, this behavior is generally implemented in code that is associated with markup. This type of code is known as code-behind.

To understand Code behind i have taken example for Button click event to show the message box. For this just double click on Button you will go to the code behind of button and write the code as follows for button click event.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
           // Show message box when button is clicked
          
           MessageBox.Show("Welcome to WPF Programming!");
        }
    }
}

OUTPUT: