His job is cleaning up after kids, of course. He apparently has a doctorate's degree in Philosophy, but not much else is known about him.
Although his appearance differs slightly between his episodes, the janitor is usually depicted with gray hair and a mustache, with a teal/blue janitorial jumpsuit. He usually carries a mop with him. He seems to have a strong dislike/fear of germs.
In You Doo, he is cleaning the school and is a cause of danger to Timmy Turner when he gets a hold of a TimmyYou Doo doll and starts sweeping it away. In this episode he says that he has Ph. D in Philosophy.