Every time you save your edits on the "Page Edit" screen, a backup of the page is taken and copied over to the Backups tab. Only the latest backup of a page is saved. There are a couple ways to restore from a backup.
To restore the page immediately after a save, you can click on the 'Undo' link in the notification bar at the top of the page.
From the "Backup" tab, you can view all the available page backups. To view/delete/restore a particular backup, click into the backup, and all the controls are in the top-right.
