Semolina Flour is typically hard durum wheat which has been ground into flour. The resulting product is high in gluten and is used for making pasta and breads. Semolina is available in coarse or fine texture flour. Look for semolina flour in your grocery or health food stores. A well known brand is Bob's Red Mill. You may want to store it in your refrigerator if you don't use it up within a month or so as it does tend to get rancid.
Durum wheat is a hard, high protein, wheat which is ground and used for making breads, pastries, cakes, etc. When durum wheat is milled, the soft fine flour made up of the wheat endosperm (protein and starchy part of the grain) is separated and used for wheat flour and the remaining coarse meal which includes the ground wheat seeds is sold as semolina. Products made with semolina flour will have a slightly coarse texture compared to typical fine wheat flour. Both of these flours are high protein and high in gluten.