On spanish.about.com they explain that you would use "que" if you can replace the English "that" with "which" and it would still make sense. So, it's a pronoun. El plan que quiere es caro. (The ...
What does "que" and "cual" mean without an accent mark? How do they compare when to each other? How do they compare to their accent-marked form?
I've seen the use of interrogative pronouns in ordinary statements, not just questions. For example, from the book "Harry Potter y la piedra filosofal": "Los Dursley se estremecían al pensar qué ...