What is self?

self2.rb

class Thing
  def self.log str
    p str
  end  
  def test_log
    begin
      self.log "does not work"
    rescue Exception => e
      p e                       
    end                         
    Thing.log "this does work A"  
  end         
  self.log "this works B"   # => "this works B"
  log "this works C"        # => "this works C"
                    
end                             
                                
Thing.new.test_log      # => #<NoMethodError: undefined method `log' for #<Thing:0x007fb0fb039db0>>        
                        # => "this does work"