Browning is where most of the flavour in baking comes from, and it is produced by two separate chemical processes that are often treated as one. Distinguishing them explains a great deal of dessert technique.
Caramelisation needs only sugar and heat
Heated past a certain point, sugar molecules break apart and recombine into hundreds of new compounds, some bitter, some fruity, some nutty, none of which existed in the original crystal.
Different sugars begin this at different temperatures, which is why a syrup made with one sugar browns at a point where another is still clear and merely hot.
The process continues as long as the heat does, so the difference between a pale caramel and a burnt one is a short window rather than a broad range.
The Maillard reaction needs protein as well
Where sugars meet amino acids from protein and are heated together, they react to form a different family of compounds, responsible for the flavour of crust, biscuit and roasted nuts.
This is why a dough containing milk, egg or flour proteins browns and smells different from a plain sugar syrup taken to the same colour.
It also begins at lower temperatures than caramelisation, which is why baked goods develop colour and aroma long before anything in them reaches caramel heat.
Water suppresses both until it leaves
Neither reaction proceeds meaningfully while a surface is wet, because evaporating water holds the temperature near the boiling point and no higher.
Browning therefore only starts once the surface has dried, which is why a covered dish stays pale and an uncovered one colours in the same oven.
It also explains why a wetter batter needs longer to develop colour, and why steam early in baking delays crust formation deliberately.
Acidity and sugar type shift the outcome
Alkaline conditions accelerate the Maillard reaction noticeably, which is why a small quantity of raising agent changes both the colour and the flavour of a baked good.
Sugars that are already broken into simpler forms react faster than table sugar, so syrups and honeys brown sooner and more deeply at the same temperature.
Substituting one sweetener for another therefore alters colour and aroma as well as sweetness, which is why such swaps rarely behave as straight replacements.
Bitterness is the intended endpoint
The compounds that make caramel interesting include bitter ones, and a caramel taken only to pale gold tastes mainly of sweetness with little complexity.
Cooking further trades sweetness for depth, since the sugar being destroyed is the sugar that would have tasted sweet.
Judging when to stop is therefore a decision about how much sweetness to give up, and it is made by colour and smell because the change is too fast to time reliably.