The diff module let’s you view the difference between node revisions. It adds a tab on every node page where you can see a list of revisions which you can control via permissions.
Before we get started go to http://drupal.org/project/diff and download the module and place it in sites/all/modules.
Then enable it at Administer >> Site building >> Modules.
Using revisions and diff is fairly simple, if you are logged in as site admin just create a story node and edit it twice with Create new revisions ticked.
After the second revision you’ll see a new tab called revisions. From this tab you can view a list of revisions and revert back at any time.
If you click on the Show diff button you’ll be able to view the difference between selected revisions.
You also have the ability to preview a diff on a node even before you save it by clicking on Preview changes.
Lastly there are a couple of options that you can set on the content types edit page at Administer >> Content management >> Content types.
If you have any feedback or need help please leave a comment.