tkinter Multiple windows (TopLevel widgets) Difference between Tk and Toplevel

Example

Tk is the absolute root of the application, it is the first widget that needs to be instantiated and the GUI will shut down when it is destroyed.

Toplevel is a window in the application, closing the window will destroy all children widgets placed on that window{1} but will not shut down the program.

try:
    import tkinter as tk #python3
except ImportError:
    import Tkinter as tk #python2

#root application, can only have one of these.
root = tk.Tk() 

#put a label in the root to identify the window.
label1 = tk.Label(root, text="""this is root
closing this window will shut down app""")
label1.pack()

#you can make as many Toplevels as you like
extra_window = tk.Toplevel(root)
label2 = tk.Label(extra_window, text="""this is extra_window
closing this will not affect root""")
label2.pack()

root.mainloop()

If your python program only represents a single application (which it almost always will) then you should have only one Tk instance, but you may create as many Toplevel windows as you like.

try:
    import tkinter as tk #python3
except ImportError:
    import Tkinter as tk #python2

def generate_new_window():
    window = tk.Toplevel()
    label = tk.Label(window, text="a generic Toplevel window")
    label.pack()

root = tk.Tk()

spawn_window_button = tk.Button(root,
                                text="make a new window!",
                                command=generate_new_window)
spawn_window_button.pack()

root.mainloop()

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{1}: if a Toplevel (A = Toplevel(root)) is the parent of another Toplevel (B = Toplevel(A)) then closing window A will also close window B.