From Wikipedia (retrieved May 14, 2015):
GitHub is a web-based Git repository hosting service, which offers all of the distributed revision control and source code management (SCM) functionality of Git as well as adding its own features. Unlike Git, which is strictly a command-line tool, GitHub provides a web-based graphical interface and desktop as well as mobile integration. It also provides access control and several collaboration features such as wikis, task management, and bug tracking and feature requests for every project.
Key features of Github involve code review, modification comments, control over branch permissions, task assignment, and customizable community guidelines. Additional features can be found “here”: https://github.com/features. Users can follow and view 75+ million projects on the site.
A tutorial video for Github can be found here.
GitHub, Inc. is headquartered in San Francisco, CA (USA).