Oh geez, you're giving me flashbacks to trying to introduce Github to non-technical students. I basically just don't even try to explain the difference, because I have them use a minimal set of git's functionality, that is, the subcommands add, commit, and push...and of course, clone and init at the beginning of the quarter.
And I teach them virtually nothing about Github other than that it's a website that you interface with git. For me, Github is a great place to get students to push their assignments and papers because I can easily comment on them...once in awhile I'll do a force push to fix something. We never do anything with forking, and I strongly discourage them from using the Github web interface to edit documents, as that is the most frequent cause of merge conflicts.
If they follow what I say, git and Github are just glorified document-uploading systems. I've almost never had a student fuck up in a way that we had to rollback something, because for the most part, people who don't use version control as their job don't need it. Version control is only particularly relevant when working in a team, doing iterative work, or deploying to multiple environments. These are things that students do not need to worry about.
And actually, even the comsci students don't really need to worry about it.
If I do attempt to explain the difference between git and Github, I usually use the analogy of git is to Github as your cameraphone is to Instagram.