The ABO blood group system classifies human blood into four main types: A, B, AB, and O. This classification is based on the presence or absence of specific antigens on the surface of red blood cells. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. Blood type is important for blood transfusions, organ transplants, and pregnancy, as incompatible blood types can lead to serious health issues. The Rh factor, another blood group system, further categorizes blood as positive or negative, adding another layer to blood compatibility.