I've recently been introduced to Git, the new most amazing thing in my development life. Git is a version control application for managing source code. Git can be used either on the command line or on a Git GUI client.
I highly recommend you use git to manage your code.
If you've heard of Git, or even if you've just searched for it, you probably came across GitHub. It's exactly what it sounds like – a place to store and share git repositories.
GitHub is a great community for open source projects, but it can get pricey if you don't want to share your code with the public community.
If your code needs to be private, there are plenty of free alternatives to GitHub. I mentioned a couple in this article.
Backup Your Code and Work Cross-Platform
The idea here is that you create a local repository and then push to a remote repository, hosted by the service you choose. This ensures you can pull the code down to a different machine if necessary, and it also makes it easy for others to view your code and help you.
If You Don't Know Git
Don't worry! It's ridiculously easy to learn. But I'm not going to teach you here because it's so well-documented on its site. Check it out here.