1 - They have similar meanings, and could be used interchangeable in many contexts. 2 - "tell of..." Is more commonly used in formal/ written/ literary English and also sounds quite old fashioned. 3 - There is a slight difference in meaning: 'tell of' is more when you you talk about the existence of something; whereas 'tell about' is generally when you are giving information about that thing/ person, etc. e.g. I was surprised when he told me of his wife, because I had never imagined that he would get married. Then when he told me of his five children I was even more astounded! He told me all about his wife: about how she was artistic and had studied art in Rome. And what a good mother she is and how well she caters for the needs of our 5 children, as well as being an amazing cook.