Is Node a superclass of all content type?
From drupal.org: An Introduction to Entities


  If you come from an OOD/P background and are trying to better
  understand what these key D7 concepts are, the following suggested
  mapping might help (albeit not strictly true from a purist’s
  perspective) :-

An entity type is a base class
A bundle is an extended class
A field is a class member, property, variable or field instance 
    (depending on your naming preference)
An entity is an object or instance of a base or extended class

  
  All these four OOD/P concepts are special in that they are
  serialisable (stored - e.g. to a database or file). Serialisation
  takes place via the Entity API.