The autosave feature is there to protect editors from losing work in the event of a power outage, accidental overwrite or a plague of locusts blocking the screen. This function can be turned on or off by Super Admins and is applied globally - i.e., all users and all pages are effected by it (at the moment, you cannot choose to enable autosave on individual pages). You can also choose the time interval between each autosave, the default being 240 seconds, or 4 minutes.
Saved versions only contain the page content - they do not contain any extra page data or options. To revert to one of the saved versions, you simply click on 'Revert' next to its entry in the list. If you have a limited database quota, you may wish to disable this feature to save space.
Reverting to autosaved versions is only available to users with roles of editor or above.