"Qué" is used when any answer is possible. It is used as a pronoun in sentences like "¿Qué quieres'", where any answer is possible, and as an adjective "¿Qué libro quieres'" when the answer can be any of the books (here the choice has been limited to books only).
Used with "ser" it calls for a definition of the subject. Thus, "¿Qué es tu nombre'" (a silly question) must be answered with something like "My name is the word people use to refer to me", and to "¿Qué son las dimensiones de la habitación'" must be answered as "The distance between its walls".
"Cuál" is used to choose among a given number of possible answers. It is used as a pronoun in sentences like "¿Cuál quieres'", when a range of options have been previously given (e.g. I've got three flavours: mint, strawberry and apple. Which one do you want'). In Spain it is not used as an adjective, so we don't say "¿Cuál libro quieres'", but this is perfectly normal in many parts of Latin America.
Used with "ser" it calls for information about the subject, so "¿Cuál es tu nombre'" is answered as "Lazarus", and "¿Cuáles son las dimensiones de la habitación'" is answered as "4 m wide and 5 m long".