You'll definitely want to read the PHP documentation on classes before diving into CakePHP
From the official reference:
Every class definition begins with the
  keyword class, followed by a class
  name, which can be any name that isn't
  a reserved word in PHP. Followed by a
  pair of curly braces, which contains
  the definition of the classes members
  and methods. A pseudo-variable, $this
  is available when a method is called
  from within an object context. $this
  is a reference to the calling object
  (usually the object to which the
  method belongs, but can be another
  object, if the method is called
  statically from the context of a
  secondary object).
Not the most facile of definitions, but this really is stuff you're gonna have to know to navigate the code in CakePHP.