Your blood type is a classification of blood based on the presence and absence of antibodies and also based on the presence or absence of inherited antigenic substances on the surface of red blood cells. There are four major groups: A, B, AB, and O. We also denote blood type with + or - to reflect whether or not you have the inherited Rh factor.
The main importance for this in medicine is helping to determine what blood would be safe to transfuse into you in an emergency. If you transfuse blood that has groups that your body is not used to it could trigger a lethal immune system reaction in which your body attacks the transfused blood.