While editing an article, you can see the different versions of an article and compare the changes between versions.  You can also restore an older version if desired.

The "revisions" meta-box on the article editing screen shows all the different versions that have been saved for that article, along with the author and the date and time the revision was saved:

Click one of the links to navigate to the compare revisions screen.  This screen allows you to compare different versions of the article, and to restore a previous version.