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A programming language.
Allows multiple people to work on the same project remotely. Allows you track when changes are made, see who made those changes and see what does changes are. Github also allows you to save a copy of your work in a virtual environment that allows you to access your work from any device.
You make a copy of that repo, allowing you to make changes without it changing the other person's original repo.
Clone a repo is saving it to your local P.C in order to be able to work on it from there.
A pull request is basically a merge request, where you request to merge your changes to the main repository