It is possible to host your own Git installation and maintain a central server for your organization. If you are a small company or an open source project, maintaining such an infrastructure can be a burden. But today, there exists a number of Git providers who can lift the burden.
GitHub is the best-known Git provider with 40 million users. Many high-profiled open source projects are hosted by GitHub. Once you have created an account at GitHub, you can explore the 85 million Git repositories currently hosted.
In modern software development, continuous integration (CI) is popular. The idea is that changes from developers are merged into the code base as soon as possible. Git's pull requests (PRs) are a way of doing so. Of course, GitHub provides a UI to create PRs and let's collaborators do code reviews. Part of a CI policy is also to run all tests automatically...