Why does toast turn brown?

Toast turns brown because heat starts a reaction between the bread's sugars and proteins. Those tiny ingredients rearrange into many new brown, tasty-smelling compounds. It is called the Maillard reaction, and it is different from burning when your toast is just nicely browned.

That same browning gives roasted coffee, cookie edges, bread crusts, and seared dinners their deep smells. If the toast stays hot too long the chemistry passes the tasty stage, and black carbon begins to form, which is why burned toast tastes sharp and bitter.