Quel/quels/quelle/quelles ? = Which/what? (French Question Words)

You already know that Que or Qu'est-ce que at the start of a question is used to ask what something is (See Qu'est-ce que c'est... and Qu'est-ce que c'est que ça/cela ? = What is... / What's that? in French,  C'est quoi, Qu'est-ce que c'est ? = What's that? (French Questions)).

Now look at these questions:

Note that the question adjective quel also means "What/Who?" (and "Which?") in French.

So when to use quel  instead of que/quoi/qui ? 

Whereas que/quoi/qui will be used to define something, to ask what they are, quel is used in the context of a choice, an alternative: what is that, relative to other elements?

Note also that Quel is an adjective, so it always relates to a noun (thing/person). Therefore, it will be used when the question "What/Who" is really "What/Which <thing/person>...?" or "What is <thing>" or "Who is <person>?":

  ->  Here the adjective quelle goes with adresse (What address?)

 

  ->  Here the adjective quel goes with acteur (Which actor?)

 

->  Here the adjective quel goes with parfum (Which flavour?)

 

  ->  Here the adjective quelle goes with personne (Which person?)

 

ATTENTION: 
Quel is an adjective, so it agrees in number and gender with the thing/person it is used with ("quel", "quels", "quelle", or "quelles").

 
Case of lequel, laquelle, lesquels, lesquelles

These are pronouns, which means they're replacing pre-mentioned nouns (i.e. "Which one(s) ...?).
Therefore, they'll be used when the thing they refer to has already been mentioned, to emphasise the question:

Note that the noun is not mentioned in the question, as lequel, laquelle... already expresses it: it would be redundant!

 

See also Quel/quelle/quels/quelles + [nom] = What a [noun] (French Exclamative Adjectives)