What

interrogative pronoun (3rd person singular inanimate), interrogative determiner / adjective.

Pronoun

In both English and French, it is considered quite lax to say:

• *You want what?
• Tu veux quoi?

But I am sure that both anglophones and francophones find these phrasings perfectly functional. Nevertheless, normatively it does seem to me that the English proposition is considered "wrong" despite its relative frequence in natural speech. Of course in school we are taught to say:

• What does she want?
   wədə(z/ʊ)
• Qu'est-ce qu'elle veut?
    kesk(ə)

There are two parts to what. The first part -- wh- -- is found in who, how, why, when, etc. and represents an "information gap" (a lack of info), the second -- -at -- is found in that and represents an object at a certain distance from the speaker. This object can also be an object of discourse, and in some cases fonctions as a grammatical subject:

• What matters is that we keep trying.
• Ce qui importe c'est que nous...
• What's strange is that I didn't get what I wanted.
• Ce qui est bizarre, c'est que je n'ai pas eu ce que je voulais.

(In grammar, "what" in these examples, is called either a fused relative pronoun, and in the case of #1 its role is simply to hold the place for a long subject to be stated later (that we keep trying), this structure is known as a WH-cleft or pseudocleft sentence (very useful despite as we shall see in the next section). In the first series of examples what was, on the other hand, an interrogative pronoun. In the following examples, it is an interrogative indirect pronoun.)

• They asked what we were doing _what .
• She imagined what the other girls were doing _what.

WH-cleft sentences

An extremely common rhetorical device to draw attention to the semantic object is the so called WH-cleft or pseudo cleft structure (in #2 below)

1. I need a dollar.
2. (What) I need (is) a dollar.
3. I need a dollar, a dollar is what I need.
4. I need a dollar. That's what I need.

The basic formula of such sentence is (what) X (be) Y. An inverted WH-cleft follows a similar pattern: Y (be) (what) X