Git is a distributed version-control system for tracking changes in any set of files, originally designed for coordinating work among programmers cooperating on source code during software development. Its goals include speed, data integrity, and support for distributed, non-linear workflows (thousands of parallel branches running on different systems). Wikipedia
try.github.io provides both resources to learn by reading or doing. A good “one stop shop” for getting started with git. They have listed documentation and online labs.
Hello World GitHub Guide
How To: Fork a GitHub Repository & Submit a Pull Request