**Git** is a "source-code management system" or "version control system" – an alternative to Subversion. It was designed for a large open-source project (Linux) and has seen broad adoption among other FOSS projects. **GitHub** is a company which provides hosted, web-based tools for enhancing Git development. For open projects like CiviCRM, its services are free.
Git and GitHub offer advantages for CiviCRM development:
* The tools are already popular among FOSS projects and web developers.
* The tools are free ("as in beer") and mostly free ("as in speech").
* They support off-line development.
* They support lightweight branching, merging, and code-review.
* They support open teams – anyone can jump-in, make changes, and share changes.
For more introductions to Git and Github, see [help.github.com](https://help.github.com/).