What is Git?
Git is a distributed version control system, or DVCS, that tracks the history of changes as people and teams collaborate on projects together. As developers make changes to the project, any earlier version of the project can be recovered at any time. In a DVCS, every developer has a full copy of the project and project history. Unlike once popular centralized version control systems, DVCSs don’t need a constant connection to a central repository.
What is GitHub?
GitHub hosts Git repositories and provides developers with tools to ship better code through command line features, issues (threaded discussions), pull requests, and code review. GitHub builds collaboration directly into the development process. Work is organized into repositories where developers can outline requirements or direction and set expectations for team members. Then, using the GitHub flow, developers simply create a branch to work on updates, commit changes to save them, open a pull request to propose and discuss changes, and merge pull requests once everyone is on the same page.
(source: GitHub Docs)