There is practically no difference, look: "czemu" - although is used mostly in informal speach (friends/family), it´s not a very colloquial word in fact, it´s just a shorter variant. I think I wouldn´t use it only in formal writing (abbreviations are rarely used in formal and professional writing) :) "Dlaczego" can be also used to emphasize a question, for example: - Czemu nie? * Po prostu nie i już. - No ale dlaczego?! pozdro :)

Thay are synonyms, but "dlaczego" sounds better, more formal, more frequent in use and "czemu" is colloquial. Meaning: dlaczego - zaimek przysłówkowy rozpoczynający zdanie pytajne: a) niezależne (np. "Dlaczego nie rozumiesz ZDS-u?"); b) zależne (np. "Nie mam pojęcia, dlaczego nie rozumiesz ZDS-u.")

There is practically no difference, look: "czemu" - although is used mostly in informal speach (friends/family) it´s not a very colloquial word in fact, it´s just a shorter variant. I think I wouldn´t use it only in formal writing (abbreviations are rarely used in formal and professional writing) :) "Dlaczego" can be also used to emphasize a question, for example: - Czemu nie? * Po prostu nie i już. - No ale dlaczego?! pozdro :)