GitHub is an online service used to track progress and changes on collaborative projects. While it was originally designed for software development, it can be used for basically any type of design project. There is also a downloadable program for local storage and tracking of files and progress.
GitHub is free for public projects (files are open to anyone), but requires a monthly charge for private projects.
GitHub offers guides via https://guides.github.com/
Codecademy offers free online courses in Git: https://www.codecademy.com/