The main tkinter module and its submodule ttk in Python 3 appear to contain identical widgets (i.e. Buttons, CheckButtons, etc.). 
So, when creating a button, one has the freedom to either use a tkinter.Button widget or a tkinter.ttk.Button. 
Do you know what is the difference between them? Why would you choose one or the other?
My opinion for beginners who are starting to learn Tkinter, is to use Tkinter widgets, because they're really easy to learn. But on the other hand Tkinter.ttk is a module designed to make Tkinter widgets look really perfectly, but is really hard to learn and there are no easy options there. Like there are no -fg, -bg. Perhaps, there are no new styles available in Tkinter. Style are only designed for ttk, and can be found in ttk.
And Tkinter widgets really don't look like other native platform widgets.
But ttk is nicer and smoother looking, and look like other native platforms.
So if you are making apps for your own private use, then use Tkinter and also use some ttk if needed, because ttk supports much cooler widgets that can change the look of your app.
And if you are making apps for public use, then go for both because Tkinter is needed for creating the window and some more important stuff, and for widgets go for ttk.
But honestly, I say use both because there are no conflicts between the two; just use them both to your advantage.
Honestly using ttk is a challenge! Because it has no Grid,Pack, Place and many other options that are normally available in Tkinter widgets. But wait!! Tkinter has those options! So use both! Try to make a nice app!
That is the real difference between the two: Tkinter widgets are more configurable, and ttk is more modern and it is configurable with styles which are really handy shortcuts. And Tkinter is like the core of the window and ttk is styling. Think of it like this:
Tkinter --- HTML,
ttk --- CSS,
Python --- JavaScript.
What is the difference between the widgets of tkinter and tkinter.ttk in Python?