When you edit a web page, at the bottom of the page is a section named Publishing Options.  If you click on this title, you will see some options, one of which is Create New Revision.  If you click this, when you Submit the page, both the original revision and the new revision will be saved.  When you edit the page again, you will see a new tab at the top of the page called Revisions.  If you click this tab, you can see the older revisions of a page, and you can view an older revision, and go back to, or Revert to that older revision, if needed. 

This will allow you to save a prior revision as you work on a page, just in case you want to go back and see what you did before or even revert to it.  I recommend that you create a new revision any time that you make any significant edits, especially deletions, just in case you might want to go back.

The revision is mainly the text and formatting information for a page.  Not everything is under revision control.  For example, photos are not saved in revisions.