Save or Update vs Merge operations in hibernate

Following is from the hibernate documentation.

saveOrUpdate() does the following:
if the object is already persistent in this session, do nothing
if another object associated with the session has the same identifier, throw an exception
if the object has no identifier property, save() it
if the object's identifier has the value assigned to a newly instantiated object, save() it
if the object is versioned (by a <version> or <timestamp>), and the version property value is the same value assigned to a newly instantiated object, save() it
otherwise update()

merge() is very different:
if there is a persistent instance with the same identifier currently associated with the session, copy the state of the given object onto the persistent instance
if there is no persistent instance currently associated with the session, try to load it from the database, or create a new persistent instance
the persistent instance is returned
the given instance does not become associated with the session, it remains detached

You should use merge when there is an  object that was detached from the session and a persistence object of it is currently associated with the session.