The Diff module lets you compare two revisions of a piece of content and see the changes. This video demonstrates the 5.x-1.x-dev snapshot version.The Revision Tags module provides an interface for tagging and browsing the various revisions of a piece of content. For example, if you have a page describing how to use a piece of software, and the software has several releases, you would write a revision of the instructions for each release and tag the revisions. That way the reader could browse to the version of the instructions pertinent to their release of the software.As noted in the comments, you need to have your content types set up to make new revisions. This is done under Administer->Content management->Content types->edit, and then check "Create new revision" which is found under Publishing options.http://www.lullabot.com/files/Drupal-Diff-and-Revision-Tags-Modules.mov