This document provides an introduction to GitHub, including what it is, why it is used, and its key features. GitHub is a file storage service for developers to save source code and files. It is based on the version control system Git and allows users to collaborate on projects. Popular features of GitHub include Wikis, Gists for code snippets, GitHub Pages for hosting websites, and extensions that enhance the user experience. The document also discusses other similar services like BitBucket and GitLab, as well as version control systems like Mercurial and Subversion.