Git is one of several “version control systems”, and the industry standard for open source software. GitHub is the specific implementation of git where a complete copy of a repository is kept online for easy remote access. It is both a technology and a workflow that allows many people to contribute to a single set of code easily, with the reason behind every change made available for posterity.
Below is a set of a few short videos that introduce the concept of Git at the most basic level.
Getting started with GitHub Desktop
GitHub is a specific implementation of Git that has an abundance of user documentation and help out on the web. GitHub has a nice desktop client to acquaint new users with the workflow, with quick access to a console for command line interface. Consult the GitHub Desktop User Guides for a walk-through type lesson.