If I am not mistaking it is very easy and clear.
The @api.one is specific for one record and can not be used for multiple records. It automaticly loops on records of of a Recordset for you. 
@api.many is for multiple records, where you can loop through it etc. It will be the current Recordset without the itiration. So any looping would need to be programmed by yourself, for whatever you want

An example of @api.one:

@api.one def _compute_name(self):
      self.name = str(random.randint(1, 1e6))

An example of @api.multi:

@api.multi def subscribe(self):
     for session in self.session_ids:
         session.attendee_ids |= self.attendee_ids
     return {}

You can read more about the @api.one and @api.multi here: http://odoo-new-api-guide-line.readthedocs.org/en/latest/decorator.html