Git is a distributed version control system (DVCS) for source code management (SCM). It's like a giant undo button for everything you've ever done on a project throughout its entire existence. Git also provides some powerful tools for collaborating with your team, browsing a project's entire history, deploying code, and so much more. Git is the version control system used for Drupal core and contributed module development and, as such, is used by most people building sites with Drupal to keep track of their client work as well. It's also the system used to track development of the Linux kernel, Ruby on Rails, Android, and many, many other projects. This is an in-depth series that starts with the basics of version control, establishes some terminology, and a base line workflow, then continues to build on that by going beyond the basics of the various Git commands to make the most out of your tools.
Joe and Blake give an introduction to the Git series giving some background on Git, and talking about the material that will be covered in the course of this series. If you aren't familiar with using the command line, you will probably want to get yourself up to speed with the basics before starting this series. We have an entire series on Command Line Basics to get you ready to go.