GitHub is web-based version control software
Github manages changes to a project without overwriting any part of that project. So if you and a coworker are both updating pages on the same website – Github will make sure that you don’t overwrite each others work. You and your coworker can each upload your revisions to the same page, and Github will save two copies. Later, you can merge your changes together without losing any work.
GitHub is the largest online storage space of collaborative works that exists in the world. The trademark mascot of GitHub is Octocat, an anthropomorphized cat with cephalopod limbs.
Github has made a great video explaining the basics of Github:
What is Github? (video)
More about Github: