Opening Pandora's box here Andy?
Carmalization is what happens to sugar under heat (above 330-F). The sugar breaks down, molecules recombines, and creates new sugars. Caramel actually has been converted from 1 sugar to about 120 different sugars.
Throw in some proteins, along with the sugars, and you get a Maillard browning reaction. Again, under heat (above 230-F) the proteins and sugars break down, recombine, and create new compounds, and turn brown. But, Maillard reactions is a tree with different branches ... and not easy to explain ... or understand. If you really want to explore Maillard reactions in depth - just google on Maillard Reactions and enjoy the ride! Food scientists have annual conventions to explor and try to understand this.
Foods that brown without heat are mainly a result of enzymes ... a result of the "death and decay" of the food.
You might want to check your local library for a copy of Harold McGee's On Food and Cooking: The Science and Lore of the Kitchen
and read the section on Maillard Reactions. Also check out the explaination that Shirley Corriher gives in her book Cookwise
Now .. outdoor grilled food gets another ingredient ... not only the reactions of the fats and sugars ... but you're also throwing in other chemicals from the smoke!
Next time you and your buds are standing around the grill you can all contemplate the complex miracle of science that is going on as a multitude of chemical reactions are going on to flavor your food!