Flour is ground cereal grain. The most commonly used flour comes from wheat but there are many others - for example, rice, chickpea, corn, rye etc.
Flour varies from hard to soft depending on the type of wheat used. Hard wheat gives a high protein, high gluten flour and is used in bread making. Plain flour is a mix of hard and soft flours, it is a general purpose flour. Soft flour comes from soft wheat, it is low in gluten and is used for cakes and pastries
Unbleached flour bleaches naturally as it ages it is higher in protein than unbleached flour.
Self raising flour has had baking powder added to it
Semolina comes from durum wheat which is the hardest wheat variety. Past is made from this flour.