The ABO blood group system is a classification of human blood based on the presence or absence of specific antigens on the surface of red blood cells. There are four main blood types: A, B, AB, and O. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. This system is important for blood transfusions, as receiving incompatible blood can lead to serious reactions. Additionally, the Rh factor, another blood group system, can further classify blood types as positive or negative, impacting compatibility in transfusions and pregnancy.