GitHub, like BitBucket, is a web-based hosting service for uploading and sharing code. It runs on the Open Source version control system Git. Developers use version control systems to organize their projects, to update and change code and to store these modifications in a central repository. Others can contribute to that repository and download and upload changes.
GitHub is literally a connection of Git and a hub. Git is a command line tool. It does not have its own user interface, though you can install one. But generally, you use it by entering short commands into the terminal/command line. The hub of GitHub is their web page, where developers store their projects and network with the GitHub community.
Many Open Source projects are hosted on GitHub so everyone can contribute, report issues and leave comments. You can also create private repositories where just a few invited people have access, but you have to pay for those. If you‘re a student, you can create private repositories for free. This is why GitHub is popular among the Open Source community. BitBucket may be cheaper and better for small teams that want to have private repositories, because there you pay per contributer no matter what kind of repository.