Blood type refers to the categorization of human blood based on the presence of genetic antigens, which are carbohydrates that attach to lipids or proteins. Antigens are categorized as A, B and Rh.
There are 4 differing blood groups which determined by the presence or absence of two antigens (A and B) on the red blood cells: Blood group A (A antigen on red cells, B antibody in plasma), group B (B antigen on red cells, A antibody in the plasma), group AB (A and B antigens on red cells but neither A nor B antibody in plasma), and group O (neither A nor B antigens on red cells, A and B antibody are in plasma).
So in addition to the A and B antigens, the third antigen is called the Rh factor -- is either present or absent and is referred to as either positive (+) or negative (-). This, along with blood groups, determine what blood type a person is. Typically, Rh negative blood is only given to Rh-negative patients while Rh positive blood or Rh negative blood may be given to Rh positive patients.