alloc = allocates a chunk of memory to hold the object, and returns the pointer
init = sets up the initial condition of the object and returns it.
2 types of methods in Obj-C
  1) instance method = a method whose execution is scoped to a particular instance of the class. In other words, before you call an instance method, you must first create an instance of the class. Instance methods are the most common type of method.
  2) class method = a method whose execution is scoped to the method’s class. It does not require an instance of an object to be the receiver of a message.
For instance methods, the declaration is preceded by a minus sign (-)
For class methods, the corresponding indicator is a plus sign (+)