about Frame.__init__(self)

[yang]

I just a newbie of python
Now I found that almost every program use Tkinter have this line

class xxx(xxx):
     """xxxxx"""

     def __init__(self):
         """xxxxx"""

         Frame.__init__(self)
         .....................
         .......


the line "Frame.__init__(self)" puzzle me.
why use it like this?
can some one explain it?
regards,
yang