There are generally no changes to the website's file system when you create pages and add and edit modules.  All of that activity occurs solely in the database.  Taking frequent database backups can allow you some basic ability to rollback changes and preserve new and old content.  

Additionally, there are some module that will keep track of your changes to content within the module.  For instance, my company's Engage: Publish module keeps track of every change to the content within it, allowing you to keep track of changes, rollback changes, and submit changes to a moderator before they go live.

DNN 5.1 has updated the HTML module so that it also has some workflow and/or approval features, but I haven't had a chance to use that extensively, so I'm not aware exactly which features and how much flexibility it offers.

Hope that helps,