Why "ce qui" and/or Quel ?

Ce qui (and its object complement ce que) is a relative pronoun which refers to something mentioned previously. Quel is an interrogative pronoun used for asking questions. In English both may, at times, be translated as "what" but French keeps them always distinct.

Here's a quick test: try to substitute "that which" for either ce qui or quel. You'll see that it only works for the former.

Ce qui est essentiel, c'est ça! -- What (that which) is essential, is this!
Quel bonheur! -- What a pleasure! Clearly, "that which pleasure" doesn't work here.

Wow, thanks, Chris!  I have been struggling with this ever since I started French two years ago.  You should see the flowchart I have trying to figure which one to use.  You made it simple.  Beaucoup beaucoup merci!