Git is the version control system adopted by OpenKore in 2016. It is the replacement for the old SVN version control on SourceForge.
OpenKore on GitHub
From a console, you can clone the most recent version of the source code by typing:
git clone https://github.com/openkore/openkore
If you'd like to contribute to the project, you can make your own fork and submit pull requests on GitHub or send patches to developers on the OpenKore Forums or IRC.
If you'd like to learn more about using Git, the official documentation has a lot of detailed tutorials.