Which vs What

When asking a question, the choice between the two depends on whether we are asking about a particular set of objects or not. In general, "which" is asking which one of these, while "what" is asking what kind.

Which ice-cream do you want, vanilla or chocolate? - Which one of these two?
What do you want for dessert? - What kind of thing do you want?

Although in most cases "what" can be used in place of "which" (Which/What newspapers do you read?),
- when we have a choice between a few things, we use which:
Which is your car, the Mercedes or the Opel?

Which of those women is your mom?
Which film are you going to see at the cinema?


- when we are asking in general, we use what:
What books do you like reading?
What advice did he give you?
What cell phone are you going to buy?


Because of this distinction, the same sentence can differ in meaning:
1) What dessert do you want?

This question can show that you are willing to make whatever dessert they ask for. Very appropriate when you want to spoil your spouse.
2) Which dessert do you want?

This means we have a few and the person needs to choose one of them.