self: Referencing to the current receiver

 Within the instance methods of a class, the pseudovariable self can be used as needed. This is only a reference to the current receiver, the object on which the instance method is invoked.

eg.

class Book
  def self.title
    true
  end

  def title
    true
  end
end

self refers to the object depends on its context. self.title in the above example will be invoked by the (current) object, Book. While title will be invoked by the object, Book.new. Therefore, to determine the value of self, you need to think around where the self resides.

There are only 2 things that change what self points to: There are only 2 things that change what self points to:

1. Explicit receiver of method call
   

   puts self #self is "main", the top-level object instance
   Object.new  #self is temporarily changed to the class Object, 
   but then back to main
   

2. class/module definition
   

   puts self #Again, self is "main"
   class Foo
     puts self #self is now Foo
   end
   #self is back to being main
   

Source:Paul Berry

So to define a method as a class method, prefix the method name with the self keyword. The scope of self varies depending on the context. Inside an instance method, self resolves to the receiver object. Outside an instance method, but inside a class definition, self resolves to the class.

Now consider this example:

class SelfStudy
  attr_accessor :name

  def self
    @name
  end

  def self.name
    @name
  end

  def self.name=(name)
    @name = name
  end

  def self.default_name
    self.name = "ClassName"
  end

  def default_name
    self.name = "InstanceName"
  end
end

Now playing with this class:

puts SelfStudy.name
#=> nil
puts SelfStudy.default_name
#=> ClassName

me = SelfStudy.new
puts me.name
#=> nil
puts me.default_name
#=> InstanceName

puts SelfStudy.name
#=> ClassName
puts SelfStudy.default_name
#=> ClassName

Please note the I just want to play up with self in the above example. So instead of defining @name in self method. As you should already know, you may simply replace it with
@name = nil