If you're already familiar with Git, you can save some reading by skipping ahead to the Creating a Git repo section. If not, here's a gentle introduction.
Even if you're the only person who works on a piece of source code (for example, Puppet manifests), it's still useful to be able to see what changes you made, and when. For example, you might realize that you introduced a bug at some point in the past, and you need to examine exactly when a certain file was modified and exactly what the change was. A version control system lets you do that, by keeping a complete history of the changes you've made to a set of files over time.
When you're working on code with others, you also need a way to communicate with the rest of the team about your changes. A version control tool such as Git not only tracks everyone's changes, but lets you record a commit message, explaining what you did and why. The following example illustrates some aspects of a good commit...