When a module is version-capable, a history is automatically stored for the content of each record of that module. With the Pages module for instance, when a record is saved the original revision is saved as a revision, and its content remains accessible by clicking on its corresponding link in the list of revisions at the bottom of the page. This list displays the revision number, the date and time of the revision, and the revision's author.
If a record is saved while viewing a previous revision, that previous revision along with any changes to its content will become the current revision. The original current revision will then become the second to last revision.
If a record is saved and the content is unchanged, a new revision will not be created.