The words porque, por que, porquê and por quê will probably torture you sooner or later as you pick up Portuguese. Since I'm all about the love, and avoid the torture - except in states where said practice is legal - I've decided to offer some tips on the matter (note to self: avoid leper joke after "tips"). So let us look at how each poorkay should be used, and their proper setttings.
Also important to remember before our journey begins is that these words are all pronounced exactly the same way. So don't go pausing after the por in por que, as opposed to an uninterrupted rendition of porque. Doing this does not convey the different meaning that pertains to each word, and will ultimately make you sound like a moron.
Let's begin with por que.
This por que translates to "Why" in English, so use him when asking a question : "Por que você chegou atrasado?" (Why did you arrive late?).
Now, moving on to porque.
This guy comes in an one piece suit - porque - and Mr. One Piece is all about providing answers, so think "because" when you see him:
Eu estou atrasado porque o meu carro quebrou (I am late because my car broke down).
The rest of the gang - Mr. Accented Por quê, and Senhor One Piece Suit Accented Porquê - do not merit my time as they get on my nerves, are rarely encountered in conversation, plus the accent looks silly. If you want to study their usage though you can read this.
I also recommend checking out the Portuguese related info offered by a user - of the forum linked above - called Macunaíma. Her submissions can be found here.
Now for a really neat trick you can use with this word: everytime you hear that typically mispronounced sound of porque being uttered by a fellow expat you should automatically say "butter".
Like in the classic ad below.
I should get a Nobel Prize for these tips (and then the leper . . .)