Each time you save a draft or publish or unpublish a page, the system saves a version of the content. To see what pages have changed recently, go to the "Reports" item near the bottom of the admin menu and choose "Site History".
On the Site History page, you can search for pages by title, action (e.g. publish), user, and date. The title on each line links to the current edit page for that page. From there you can make more edits, preview the current draft, and go to the page's full history by clicking on the Page History link in the edit page header.
To see a list of each time the current page has changed, click the "History" link at the top of the edit page. Or, if you are on the Sitemap, there is a link to each page's history under the "More" dropdown menu.
The history for each page includes a list of all drafts, as well as entries each time the page is published or unpublished. You can compare each version with the version before and after it. If you would like to revert the current page to a previous version, click the "Review this version" link on the draft you would like to recover.
"Review this version" will take you to an edit page with the chosen version of the page. Since this is not the current draft of the page, you will see a brown band at the top with the version information (date and who created it) and the actions bar at the bottom of the page says "Replace current draft" rather than the usual "Save draft".
At this point you can edit and preview as usual - then save draft or publish your recovered version.