Git is very popular source control software which easily allows groups of people to work on the same documents or files at the same time.
Along with Git there is a very popular way of hosting your source control files, this is Github.
Github allow you to have an unlimited number of public repositories, which means once you commit your files to Github they will be open to anyone to fork (make a copy) this repository. There are premium versions of Github which allow you to have private repositories to stop anyone getting at the code before it's ready to be released.
Git isn't hard to learn how to use, it's actually very easy to use which is one of the reasons why it's become so popular.
But if you haven't used any form of source control before it can take a while to get your head around how it all works.
This is where this web applications can help. Code School have developed a nice application that will teach you the basics of using Git not by reading about it but by doing it.
Running this demo you will create a repository, add a file and comment your changes to Git.
Try it for yourself and learn how Git works.