Flour and grains
Protein content determines how much gluten a flour can form. Bread flours typically sit higher in protein, giving dough the strength to hold gas through a long fermentation. Pastry and cake flours are lower, for tenderness.
Wholegrain flours include the bran and germ, adding flavour, colour and nutrients but also absorbing more water. Heritage and ancient grains such as spelt or einkorn bring distinctive flavours with a more delicate gluten.
Freshness matters too: wholegrain flour contains natural oils that can turn stale over time, so it is best used within a few months.