Git is a distributed version control system for software development^1.
Version control allows you to manage code versions and changes to code. You take snapshots, so to speak, of every change you make.
Distributed, in this context, means multiple developers can write code on each of their development computers and merge the changes, usually to a centralized git server.
GitHub runs a web-based git server at GitHub.com so teams can store their code in a central location on the internet.
Ok, technically, even though It’s often used for software source code, it can be used for any set of files and directories.