Whats the difference between Python2 and Python3 ?
For those kind of questions, Google is your friend
Here is what the official python wiki has to say about that...
What are the differences?
Short version: Python 2.x is legacy, Python 3.x is the present and future of the language
Codecademy is using a few versions of 2.7.x which are all more than adequate to get introduced.
Both version 2.x and 3.x will cohabit on your desktop if you wish to install both but if all you want to do is experiment while you do the python lessons here, there is no need to install anything: You can just head to the lab!
When you are done with the course, if you want to go deeper, you can just install 3.x branch and grab a book and/or read the official documentation.
Good luck to you!
Codecademy Python vs Python 3.x
While Python 2.7 and Python 3 share many similar capabilities, they should not be thought of as entirely interchangeable. Though you can write good code and useful programs in either version, it is worth understanding that there will be some considerable differences in code syntax and handling.
The biggest difference between Python 3 and Python 2 is not a syntactical one, but the fact that Python development 2.7 will lose continued support in 2020 and Python 3 will continue to be developed with more features and more bug fixes.