Each time you save changes to any object, Author records who made the changes and when the changes were made. This information can be used to find objects, and provides you with a complete audit trail of changes.
Certain events also create history records. A history record works as a "snapshot" of an object. You can view the history compared to the current revision, to a previous revision, or any two revisions to each other. You can also revert to a previous revision provided the history record is available.