GitHub hosts your repositories on the web, while Git is the software that tracks changes on your machine. They are often confused but do different jobs.
Git handles commits, branches, and history locally. GitHub stores a copy of your repository remotely and adds pull requests, issue tracking, access control, and code review. GitLab and Bitbucket play the same role.
The distinction matters because people blur them. You can use Git alone with no account anywhere, pushing to a plain server. You can also swap GitHub for a competitor without touching a single Git command. Treating pull requests as a Git feature, when they are really GitHub's addition, leads to confusion about what each layer provides.
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