The Maillard reaction is responsible for the browning of meat, bread crusts, coffee, and dozens of other foods. It's a cascade of reactions between amino acids and reducing sugars, producing hundreds of distinct flavor and aroma compounds.
It requires heat above ~140°C (284°F). This is why boiling doesn't brown food — water's boiling point (100°C) is too low. You need to get the surface dry and hot enough.
The flavor difference between a seared steak and a boiled one is almost entirely Maillard products. "Caramelization" (pure sugar browning) is a separate reaction, though they often occur together.