Jr. = junior = adjective = the younger. The son has the same first name as his father. For example, his father is John W. Smith. The son is John W. Smith, Jr. (Today, some people do not use a comma after "Smith.") May I speak with the junior Mr. Smith? Sorry, but John W. Smith, Jr., is not here today. Would you like to speak with John W.Smith, Sr.? (Sr. = senior = the older)