He is a parody of Dick Tracy that features a hopelessly incompetent police detective, speaking in a style similar to Columbo.
After a crime was committed, Dan would appear with two police officers and make a horrible mess of things. In one episode, a wealthy woman's jewelry was stolen. When the butler walked by Dan covered in pearls and other gems, he let him go by while investigating a statue.
His famous line was "I'm Detective Dan" in a thick, cliché private eye voice. He would sometimes add on to the line depending on the situation, such as when he walked in wearing a trenchcoat and boxers, and said, "I'm Detective Dan, and I'm not wearing any pants."
He had one daughter (Amanda Bynes) who was clearly more intelligent than him when it came to his house being robbed. After Detective Dan leaves, someone would quote "What a moron."