Human blood groups are classified based on the presence or absence of specific antigens on the surface of red blood cells. The most well-known blood group system is the ABO system, which includes four main types: A, B, AB, and O. Each type is determined by the specific antigens present; for example, type A has A antigens, while type B has B antigens. Another important blood group system is the Rh factor, which indicates the presence (+) or absence (-) of the Rh antigen. Together, these systems help determine compatibility for blood transfusions and organ transplants, making them crucial for medical procedures.