what is self?

self1.rb

module Logger
  def log str
    p str
  end  
end

class Thing
  extend Logger
  
  def test_log
    begin
      self.log "does not work"
    rescue Exception => e
      p e                       
    end                         
    Thing.log "this does work"  
  end                           
                                
  def who_am_i                  
    p "self is #{self}"         
    p "Thing is #{Thing}"       
  end                           
end                             
                                
Thing.new.test_log      # => #<NoMethodError: undefined method `log' for #<Thing:0x007fb0fb039db0>>        
                        # => "this does work"                                                      
Thing.new.who_am_i      # => "self is #<Thing:0x007fb0fb038ac8>"
                        # => "Thing is Thing"