A blood group system is a classification of blood 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 system. The ABO system categorizes blood into four main types: A, B, AB, and O, depending on the antigens present. The Rh system, on the other hand, determines whether blood is positive or negative based on the presence of the Rh factor antigen. Understanding blood group systems is crucial for safe blood transfusions, organ transplants, and pregnancy, as incompatible blood types can lead to serious health complications.