(Hinweis: Für eine deutsche Antwort, siehe hier.)
I don't think there is much of a difference. Perhaps jetzt is more often used in spoken language, while nun is more often written than spoken, although there may well be regional differences.
If you must make a difference: jetzt means "right now, this instant" where the meaning of nun is closer to "presently" or "by now". It's somewhat broader and can be used as a filler as well (in a fairy tale, say):
Nun da alle Königssöhne in die Ferne gezogen waren, blieb nur noch ... (Now that all of the king's sons had wandered off, only ... remained ...)
Jetzt can never mean "well", not even colloquially, whereas "nun" is often used as an interjection:
Nun, das kommt darauf an ... (Well, that depends ...)
There are also some fixed expressions where you can't really exchange one for the other:
Nicht jetzt! Nun denn. Jetzt oder nie. Wann, wenn nicht jetzt? ... im Hier und Jetzt.
For the most part nun and jetzt are synomyns, though. There's another thing: if you are talking about the current state of something, use jetzt:
Die ehemalige "Gebrüder Heindl Gmbh" (jetzt: Heindl & Co. KG) ...