How to undo (almost) anything with Git
A handy guide to fixing common mistakes with Git from Joshua Wehner:
One of the most useful features of any version control system is the ability to “undo” your mistakes. In Git, “undo” can mean many slightly different things.
When you make a new commit, Git stores a snapshot of your repository at that specific moment in time; later, you can use Git to go back to an earlier version of your project.