Flour and Gluten
Wheat flour contains proteins called glutenin and gliadin. When flour is mixed with water, these begin to combine forming strands of a more complex protein, gluten. Kneading increases gluten formation and makes the gluten stronger and more elastic. It is the stretchy, elastic quality of gluten that allows dough to expand as the yeast causes it to rise, and conversely to "pull back" so that the dough is not stretched so thin that all the gas bubbles burst.
Flour that has a high proportion (12-14%) of gluten-forming proteins is usually labelled "strong" or "bread flour". This sort of flour is ideal for making traditional sandwich loaves in tins, as the high gluten content will allow for a high rise. Strong flour is not suitable for cakes and biscuits as the gluten content would make them tough. Flatter breads such as ciabatta or pizza can be made with a mixture of strong flour and lower protein flour, labelled "plain" or "all purpose", giving them a more tender texture. It is worth experimenting with different blends to find a combination you like.
Wholemeal flour complicates the matter a little: the flakes of the outer layer of the wheat grain, called bran, act like tiny razor blades which cut through the strands of gluten, reducing the dough's rising ability. Also, since the outer layers of the grain do not contain any gluten forming protein, less gluten will be produced for a given amount of flour, compared with white bread. This is why traditionally made wholemeal bread has a denser texture than white bread.
Of course, other grains apart from wheat can be used in bread making. Rye flour has a distinctive flavour which works particularly well in sourdough bread. It has a tendency to become gummy if kneaded for a long time, due to the presence of compounds called pentosans. It also has a lower gluten content than wheat, making it more digestible for some people but reducing its ability to make a high-risen soft-textured loaf.
Spelt is an ancient relative of modern wheat which is gaining popularity amongst bakers for its slightly nutty flavour. It also has a lower gluten content than wheat, and is often combined with strong bread flour to make a well-risen loaf. Some Italian breads use a proportion of durum wheat flour - durum wheat is a hard, high protein variety of wheat that is used for pasta making, but most of the protein is not of the right type for making gluten