"Bread" refers to the substance. "Loaf" refers to an American Football shaped object MADE of bread. "You need sliced bread to make a sandwich." "Can you buy a loaf of bread next time you go to the store?"

A loaf is a shape - often rectangular, but sometimes round. It is usually around maybe 1/2 a kilogram. Bread is the material. So you have a loaf of bread. You can also have bread rolls, or a slice of bread. Bread rolls are like a loaf, except much smaller.