this is a much better list it seems so posting this will delete the other one
Github is basically just a fancy front-end and hosting provider for Git. If you understand what Git does, then most of Github will become self-explanatory.
Git is a "distributed revision control system.". You can think of a revision control system as edit->undo/redo on steroids. As you work on a project, you can "commit" the current state. The entire commit history is stored, and you can return to any commit in that history.
What makes Git "distributed" is its specialization towards branching and merging, so many people can work on a project and rectify their changes. Also, that everything needed for it to work is stored locally. Network connections are only required when pushing/pulling commits from somebody else.
If you want the long answer, https://git-scm.com/book/en/v2
