Subversion is a free/open-source version control system. That is, Subversion manages files and directories, and the changes made to them, over time. This allows you to recover older versions of your data, or examine the history of how your data changed. It is a general system that can be used to manage any collection of files.
Subversion is a great tool for colaborative work. Whenever there is need for me to work with other people sharing files (software development or text writing, mostly) I set up a Subversion repository and grant access to those who need it.
This page gives minimal instructions to get access to my repositories.
Let us say that you want to have access to the repository named transmogrifier.
To get access to the repository, send me your public ssh key. This is usually the contents of ${HOME}/.ssh/id_dsa.pub or ${HOME}/.ssh/id_rsa.pub
To get a local working copy of repository transmogrifier, run (after I have enabled your access)
svn checkout svn+ssh://renato@fradim.inf.ufpr.br/transmogrifier
After working in your local copy of directory transmogrifier, to upload the resulting new version, run (at ./transmogrifier)
svn commit --message "<explanation of your changes>"