You may say that a father and a dad are similar to each other. There are some differences though with how these terms will be used. When you say “father,” this is someone whose sperm has been used in order to create a human being. When a man is able to create a life because of his sperm, this means that he has fulfilled his duty as a father.
When you say “dad,” this is a person who makes it a point to provide for his family. He tries to make sure that all of the needs of his family members are met. He wants to make sure that his children will have a roof over their heads, and so much more.