Professional directories most often list members of specific professions such as medicine or law. They often include biographical information and a short paragraph of the person's history within the profession. Like professional directories, organizational directories are lists of a specialized nature such as names of university alumni or members of fraternities. They may include addresses as well as dates of birth and death.
Once you locate an ancestor in these directories, you may be able to read a history about his experiences in that profession or a small biography about his life.