Strong white flour (also known as bread flour) is made from hard wheat varieties. It has a high gluten content which makes it perfect for bread and pizza dough as it allows the dough to rise properly. Strong white flour also helps baked good keep their shape while in the oven.
In most big supermarkets you will be able to find both wholemeal and white strong bread flour.
Soft wheat flours like plain flour have a lower gluten content which makes it better cakes, biscuits and pastries like croissants.