What does it mean?

[anafor2004]

When we define a class in a header file, i write down this code "" Ui::Dialog ui; ""
what does it mean?
If we have Two form first one is MainWindow the second one is Dialog window, these forms all classes contains these code why?

class MainWindow : public QMainWindow, public Ui::MainWindow
{
    Q_OBJECT
public:
 
    int i;
    QTranslator translator;
    //Dialog::Dialog *a=new Dialog();
	 MainWindow(QWidget *parent = 0);
	~MainWindow();
 
 
private:	
    Ui::MainWindow 		ui;
 
 
    void languageChange(int);
 
protected:
		void changeEvent(QEvent* event);
		void keyPressEvent(QKeyEvent *e);
public slots:
	void english();
	void turkce();
	void deutsch();
	void reset();
	void sayfa();
 
 
};
 
class Dialog : public QDialog , public Ui::Dialog 
{
    Q_OBJECT
public:
	Dialog(QWidget *parent = 0);
 
private:	
    Ui::Dialog		ui;
 
   public slots:
		void gizle();
 };

class MainWindow : public QMainWindow, public Ui::MainWindow
{
    Q_OBJECT
public:
	
    int i;
    QTranslator translator;
    //Dialog::Dialog *a=new Dialog();
	 MainWindow(QWidget *parent = 0);
	~MainWindow();
	

private:	
    Ui::MainWindow 		ui;
    
    
    void languageChange(int);
 
protected:
		void changeEvent(QEvent* event);
		void keyPressEvent(QKeyEvent *e);
public slots:
	void english();
	void turkce();
	void deutsch();
	void reset();
	void sayfa();
 
    
};

class Dialog : public QDialog , public Ui::Dialog 
{
    Q_OBJECT
public:
	Dialog(QWidget *parent = 0);
	
private:	
    Ui::Dialog		ui;

   public slots:
		void gizle();
 };

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thanks