When you make changes to the blocks on a page in concrete5, those changes are saved as a new version when you leave edit mode. You can see all the versions of a page by putting it in Edit Mode and then clicking Versions:
Only one version of a page is "active" at any given point, it is the bold one in the list. If you have made changes and saved them with Preview instead of Publish, you will see the most current version (version 4 above) when logged in as an editor, but someone without editing rights to this page will see the active version (version 3 above.)
To change which version is active, simply click the checkbox next to it and click Approve.
To see what a specific version looks like, click the underlined link and a copy of the page will show up in an overlay window.
To see the differences between versions click the checkboxes of the two version in question and then click Compare. While not perfect, this view will try to highlight things that have been removed in red, and added in green.
You can remove old versions as well, but we generally recommend not doing this. While it is nice to keep a clean house, hard drive space is cheap and on anything but the largest of sites there's really no performance impact on keeping old versions around. Removing old versions can have unexpected results, for example a survey added to an older version of the page which is no longer displayed will be removed from the reporting part of the Dashboard if you delete every version it appeared on.
If you are unable to go into edit mode on a page (like if a block breaks it), you can go to to the sitemap in the dashboard, click on the page name, click Versions and roll back to a previous version.