If a library is configured for versioning, you can see the version history for the current page by selecting Page History from the Page Ribbon.
The Page History page allows you to compare versions to see what changed between versions. You can also use it to roll back to a previous version. Doing so does not delete the current version. Instead SharePoint will create a new version that is identical to the version to which you are reverting. As a result, you preserve the history of the page including the rollback action.