¿Qué? v. ¿Cuál?

The words qué and cuál both mean "what" or "which", but aren't interchangeable. The main rule of thumb is that qué is used to ask about an object directly, and is followed by a noun almost always:

¿Qué cosa quieres hacer? What(which) thing do you want to do?
¿Qué estación es? What(which) station is it?

The word cuál is used any time that there is a selection or something from which to choose, or there is no noun following:

¿Cuál es la fecha de hoy? What(which) is today's date?
¿Cuál es su clase favorita? What(which) is your favorite class?