Fantastic question! A blood type tells you what kind of surface is on a particular person's red blood cell. The structures on the surface of red blood cell that determine blood type are called antigens. If your red blood cells have type A antigens, then you have type A blood. Type B antigens = type B blood and if you have a combination of both type A and B antigens then you have type AB blood. If your red blood cells do not have any antigens at all, then you have type O blood. Humans and our close animal relatives the apes (chimpanzees, bonobos and gorillas) all have these same ABO antigens on our red blood cells. Other animals have antigens on their red blood cells that are different from humans and the apes, so they have different types of blood (not the ABO typing like we have).