GitHub has become the most popular website for open source projects, thanks to the migration of some major projects to Git (for example, Eclipse) and new projects adopting it, along with the introduction of the social aspect of software projects that piggybacks on the Facebook hype. The following diagram shows the GitHub collaboration model:
The key aspects of the GitHub workflow are as follows:
Each developer pushes to their own repository and pulls from others
Developers who want to make a change to another repository, create a fork on GitHub and work on their own clone
When forked repositories are ready to be merged, pull requests are sent to the original repository maintainer
The pull requests include all of the proposed changes and their associated discussion threads
Whenever a pull request is accepted, the change is merged by the maintainer and pushed to their repository on GitHub
The preceding workflow works very effectively for most open source projects; however, when the projects gets bigger and more complex, the tools provided by GitHub are too unstructured, and a more defined review process with proper tools, additional security, and governance is needed.
In May 2012 Linus Torvalds, the inventor of Git version control, openly criticized GitHub as a commit editing tool directly on the pull request discussion thread: "I consider GitHub useless for these kinds of things. It's fine for hosting, but the pull requests and the online commit editing, are just pure garbage" and additionally, "the way you can clone a (code repository), make changes on the web, and write total crap commit messages, without GitHub in any way making sure that the end result looks good." See https://github.com/torvalds/linux/pull/17#issuecomment-5654674.
Gerrit provides the additional value that Linus Torvalds claimed was missing in the GitHub workflow: Gerrit and GitHub together allows the open source development community to reuse the extended hosting reach and social integration of GitHub with the power of governance of the Gerrit review engine.