what's the difference between mainloop and app.mainloop and root.mainloop?

[luofeiyu]

#code1
from Tkinter import *       
class Application(Frame):     
    def __init__(self, master=None):
        Frame.__init__(self, master)   
        self.grid()                    
        self.createWidgets()

    def createWidgets(self):
        self.quitButton = Button ( self, text='Quit',
            command=self.quit )               
        self.quitButton.grid()         
        
app = Application()
app.master.title("Sample application") 
app.mainloop()

#code2
from Tkinter import *       
class Application(Frame):     
    def __init__(self, master=None):
        Frame.__init__(self, master)   
        self.grid()                    
        self.createWidgets()

    def createWidgets(self):
        self.quitButton = Button ( self, text='Quit',
            command=self.quit )               
        self.quitButton.grid()         
        
app = Application()
app.master.title("Sample application") 
root=Tk()
root.mainloop()

#code3
from Tkinter import *       
class Application(Frame):     
    def __init__(self, master=None):
        Frame.__init__(self, master)   
        self.grid()                    
        self.createWidgets()

    def createWidgets(self):
        self.quitButton = Button ( self, text='Quit',
            command=self.quit )               
        self.quitButton.grid()         
        
app = Application()
app.master.title("Sample application") 
mainloop()

what's the difference between mainloop and app.mainloop and root.mainloop?
do they have different meaning?
which one is right or the best in code1,code2,cod e3??