In addition to Python 2 and Python 3, there is more than one version of each.
First of all, there are the Pythons which are maintained by the people gathered around the PSF (Python Software Foundation), a community that aims to develop, improve, expand, and popularize Python and its environment. The PSF’s president is Guido von Rossum himself, and for this reason, these Pythons are called “canonical”. They are also considered to be “reference Pythons”, as any other implementation of the language should follow all standards established by the PSF.
Guido van Rossum used the “C” programming language to implement the very first version of his language and this decision is still in force. All Pythons coming from the PSF are written in the “C” language. There are many reasons for this approach and it has many consequences. One of them (probably the most important) is that thanks to it, Python may be easily ported and migrated to all platforms with the ability to compile and run “C” language programs (virtually all platforms have this feature, which opens up many expansion opportunities for Python).
This is why the PSF implementation is often referred to as CPython. This is the most influential Python among all the Pythons in the world.