JDesktopPane is a Swing container that provides a workspace for arranging and managing internal frames. An internal frame is a subwindow that exists within the JDesktopPane and can be moved, resized, and iconified. It is often used in desktop applications to create a multitasking environment.

Here's an example of how to create a simple application that uses JDesktopPane and JInternalFrame.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class DesktopDemo extends JFrame {

    private JDesktopPane desktop;

    public DesktopDemo() {
        super("Desktop Demo");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Create a desktop pane and set it as the content pane
        desktop = new JDesktopPane();
        setContentPane(desktop);

        // Create a menu bar with a menu that allows the user to create new internal frames
        JMenuBar menuBar = new JMenuBar();
        JMenu menu = new JMenu("File");
        menuBar.add(menu);

        JMenuItem newItem = new JMenuItem("New");
        newItem.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                createInternalFrame();
            }
        });

        menu.add(newItem);
        setJMenuBar(menuBar);

        // Set the size and location of the window
        setSize(500, 500);
        setLocationRelativeTo(null);
    }

    private void createInternalFrame() {
        JInternalFrame frame = new JInternalFrame("Internal Frame", true, true, true, true);
        frame.setSize(200, 200);
        frame.setVisible(true);
        desktop.add(frame);
    }

    public static void main(String[] args) {
        DesktopDemo demo = new DesktopDemo();
        demo.setVisible(true);
    }
}

This creates a window with a menu bar that allows the user to create new internal frames. When the user clicks the "New" menu item, the createInternalFrame method is called, which creates a new internal frame and adds it to the desktop pane.