Semolina is made from the endosperm (the core or heart) of Durum Wheat, coarsely ground. It is then sifted. The coarser material becomes Semolina, the finer stuff becomes Durum Flour. The Durum Flour is the texture of other flours, whereas the Semolina is closer in texture to granulated white sugar or coarse cornmeal.
Semolina is used to make pasta, couscous and speciality breads. Durum Flour is preferred for noodles.
In the south of Italy, in areas such as Sicily, finely-ground Semolina is often used along with white flour 50/50 to make bread. The bread ends up quite dense, but very flavourful.
Elsewhere in Italy, though, it is only used for pasta. It is not used in pizza dough.
Italian semolina textures may be different from North American ones. Most American semolina is very coarse, as it is "chipped" rather than ground. European milling machines for Semolina, however, can be set to different textures based on the specifications of who has ordered it.
If you have bought coarse North American semolina and suspect (or know) that a finer one is actually wanted in a recipe, such as in Sicilian baking, then you can whizz it in a blender.
Small portions of Semolina, up to 25%, can be swapped in for regular flour in bread. But beyond that, the loaves won't rise well, and will be heavy. Higher proportions can be used, up to 60%, if the bread is made with a sponge dough (i.e. biga) method.