"Which" is used when a person is making a choice between just a couple of options. "Which shoes should I buy?" (The speaker is probably trying to decide between two or three pairs.) "What" and which can both be used when there are too many choices to list. For example, in English we would say, "What city are you visiting when you go to California?" or "Which city...." But I believe in Spanish, you would have to say, "Cual cuidad ...." "What" is also used when the answer isn't related to a set of choices. For example, "What did John say to you just now?" "What" is used much more often than "which."

Normalmente which es cual, y what es que. Por ejemplo; Which shirt should I wear today? What time does the wedding start? Aunque podemos usar "what" en el primer pregunta como, "What shirt should I wear today?" este forma no es commun por que "what" es indeterminado y "which" es determinado