Difference in "why"

What's the difference between "po co" and "dlaczego." Both mean "why" (or so I've read), but which is used for which cases and what reasons? Please and thank you! :)

po co -> what for
dlaczego -> why

Not exactly.

Po co? is What for?
Dlaczego? is Why?

PS. Ooops, Mom was first. Sorry.

Dlaczego = Why

Po co = For what purpose

Nie chcę iść dziś wieczór do kina. - Dlaczego? = I don't want to go to the movies tonight. - Why not?/How come?

Chciesz uczyć się języka rumuńskiego? Ale po co? = You want to learn Romanian? But why?/What(ever) for?

Does this make things clearer?
-:)

Yes, thank you. Now I know how to use it.

po co sounds like argument (against). It's safe to use dlaczego when you are beginner ;)

Yes, much! Thank you :))

You all forgot czemu?

[I found this imitation of Chekhov somewhere on internet and thus dully translated it because it fits the topic]

- Czegój??
- You stole the bolts, you fool! And the train was derailed! People were killed!
- Oh ... it was so. So it was, sir. They were killed. God rest their ....
- Silence! Well, now that you have confessed to the crime, you boor, you'll be sent away.
- Czegój?
- You'll go to Siberia, you wretch !!
- Oh, but I'm innocent, sir. We took these fittings just like that, because they were driving that locomotive up and down and the cows stopped giving milk, so we well, this and that ...

- Silence, you yokel!
- Czegój ?????