SVN, aka Subversion is regards as the “time machine” for the software developer. To make things for easy understanding, suppose you are “working” in the Civilization III:

• Save the Game — svn commit commit the change to the repository.
• Load the Game — svn revert You can always rollback to the previous working copy.
• Load the Game 200 years before for the failure to build Great Library — Yes, you can svn merge the difference of now and history, then bring the history to the current working copy.
• //Sigh.. I have to fixed my mistake 200 years before, then redo almost the same things else — For SVN user, you can diff the modification from 190 years before to now and applied to the above copy.
• Damn, I lost all the .SAV files! — You can leave the SVN repository in the remote host, like sourceforge.net or tigris.org.

This is what we call Version Control. PyAWS is under the SVN version control at sourceforge.net.