From Joomla! Documentation
GitHub is a development platform inspired by the way you work. From open source to business, you can host and review code, manage projects, and build software alongside 50 million developers.
The Joomla CMS code base and many other aspects of the project are managed using GitHub as a version control and project management tool.
Create an account
To start using GitHub, create an account at https://github.com
Getting Started Guides
GitHub have a comprehensive Getting Started section which new users would be well served to familiarize themselves with.
The Joomla CMS Repository
Joomla's main CMS Repository is where you can assist with tasks like patch testing, contributing code and reporting issues you find with the system.
There are many other repositories managed by Joomla which you can either browse in GitHub, or private repositories you may be invited to join when you are a member of a Joomla Team.
In participating in the Joomla project as a coder, patch tester or other contributor, there's a variety of terms that are related directly to using GitHub. These resources should assist you with understanding this terminology: