iOS uses the design patter Model-ViewController (MVC).  In MVC every object is either:

A View Object

Visible to the user (e.g. buttons. text fields, etc)
Make up an applications users interface

A Model Object

Hold Data and know nothing about the user interface

A Controller Object

Managers of an application.
Configures the views the user sees and makes sure the view and model objects keep in sync.

 

UK Electronic Designer