why does toast smell and taste different than bread? Besides the obvious fact that toast has less moisture due the toasting process, how is it different than regular bread?  Why does bread taste different in toasted form than it regularly does?
Mailliard reactions. I'm going to guess there's already another question for this one and this will prove a duplicate, so low-effort answer.

http://en.wikipedia.org/wiki/Maillard_reaction

Basically (and I don't claim to know any better than that) it's alterations in the proteins brought about at elevated temperatures (there may also be carmelization, but that's a different-yet-similar process.)