The Difference Between "Cuál" and "Qué"

Although "cuál" and "qué" mean roughly the same thing ("what"), they can't be used interchangeably.  

The main difference is that "cuál" also means "which", or "which one", when faced with a selection or a list; however, it means "what" when the verb ser follows.  

"Qué" means an interrogative "what" when asking for a definition of something, or posing a question if the verb ser doesn't follow.  HOWEVER, when you are asking what the definition of a word is, or what an object is that is unknown, you use the interrogative qué.

It might sound complicated, but you'll see a pattern!  Let's look at some examples.

Using cuál, when posing a list or selection: 

¿Cuál prefieres, la camiseta azul o la blanca?  Which do you prefer, the blue shirt or the white?  

Using cuál, when it means "what" (noticed it's followed by the verb ser):

¿Cuál es la capital de Bolivia?  What is the capital of Bolivia? 

Using qué, when asking a question:

¿Qué quieres?  What do you want?

Following qué with ser, when begging the meaning of a word, object, or abstract thought:  

¿Qué es una bodega?  What is a bodega?  

A final note: some advanced Spanish speakers might wonder why the accent falls on the monosyllabic words when they aren't needed according to the rules of Spanish diacritical marks.  Well, they are an exception according to the Real Academia Española, the governing body of the Spanish language throughout the world.  Not only do they distinguish themselves as question starters, they also need to be differentiated from other uses of the aforementioned words.  For example, we use non-accented que in subjunctive clauses: 

Necesito que pongas la mesa.  I need you to set the table.