Roasting causes a chemical reaction called the Maillard Reaction, which is essentially a fancy scientific name for the process of “browning”. The Maillard Reaction produces melanoidins, which create a brown color and toasty flavors. Maillard Reaction is the same process which gives bread crust, seared meat, and toasted marshmallows their distinctive flavors.
Roasting can produce flavors in malt akin to toast, bread, biscuits, honey, nuts, toffee, coffee, chocolate, and even ash. The exact colors and flavors produced by roasting depend greatly on three things: grain moisture, roasting temperature, and roasting time.
Grains can be roasted at temperatures ranging from 200°F to 400°F. The higher the temperature, the darker the roast.