ABO and Rhesus (Rh) are the two most important blood group systems.
Within the ABO system, there are four main groups — A, B, O, AB — determined by the presence or absence of A and B antigens on the surface of red blood cells. If you are group A, this means A markers are present on the surface of your red cells; group AB means both A and B markers are present; while group O means there are no A or B markers on the surface of your red cells.
Within the Rh system, depending on the presence or absence of the RhD antigen, you can be either RhD positive (+) or RhD negative (–).
This results in a total of eight main blood groups: A+, A–, B+, B–, O+, O–, AB+ and AB–.
Fun fact: There are close to 40 human blood group systems recognised by the International Society of Blood Transfusion (ISBT), with ABO and Rh being just two of them.