What is intvar?

[W. eWatson]

I'm looking at someone's code in which invar() is used fairly often. 
Apparently, it's a Tkinter method. Here's a use:
     def body(self,master):
         self.title("Display Settings")

         self.colorVar = IntVar()
         Radiobutton( master, text="Gray Scale",
                      value=1, variable=self.colorVar).grid(row=0, sticky=W)
         Radiobutton( master, text="Pseudo Color",
                      value=2, variable=self.colorVar).grid(row=1, sticky=W)
         ...

What is the need for this use? It looks like some sort of initialization for 
a widget.

I've scoured the internet with Google, and have yet to find a simple 
explanation of what it's used for.
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                                W. eWatson

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