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
@api.model is basicly a converter from the old API to the new API. (Odoo V7 -> Odoo V8) Its for migrating code.
@api.depends will trigger the call to the decorated function if any of the fields specified in the decorator is altered by ORM or changed in the form..

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 {}

An example of @api.decorator:

@api.returns('res.partner')
def afun(self):
    ...
    return x # a RecordSet

An example of @api.model:

@api.model
def afun(self):
    pass

An example of @api.depends:

@api.depends('name', 'an_other_field')
def afun(self):
     pass

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