why does toast smell and taste different than bread?
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.)