Pages » Page Revisions
The Revisions module shows any backup copies of your page. Each time you click Save Draft or Update, a revision is saved. WordPress.com will store the last 25 revisions for each page on your blog. Revisions allow you to look back at the recent changes you’ve made and revert to an earlier version if necessary.
Note: The module will not show until you have made some revisions. Also you can’t manually delete page revisions, but the oldest revision is automatically deleted if there are more than 25 revisions.
To compare Page Revisions, select a revision in the Revisions section under the text editor. You’ll be able to tell at glance what was removed from and/or added to the revision:
To restore a previous version of a page, select the desired revision in the Revisions section under the text editor, then click on the blue Restore This Revision button: