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 systems are the ABO system and the Rh factor. The ABO system includes four main blood types: A, B, AB, and O, determined by the presence of A and B antigens. The Rh factor indicates whether blood is positive (+) or negative (-) based on the presence of the Rh antigen. Blood type compatibility is crucial for blood transfusions, organ transplants, and pregnancy, as incompatible blood can lead to serious health complications.