An article published by Business News This Week explains the difference between Git and GitHub.
This is not so much an apples-to-apples comparison, as an explanation why the two terms shouldn’t be used interchangably. Git is a program (developed by Linux creator Linus Torvalds) that allows for decentralized version control for code, although in recent days Git is being used to manage much more. Unlike older applications such as Subversion, which rely on a central repository and allow only one user to work on a file at any given time, Git allows many users to make changes in parallel, which are all “merged” into a main copy before deployment. GitHub offers a platform that can serve as this main copy (this is an oversimplification, but sufficient for a basic understanding) while adding features such as discussions, code review, and task management.