The ABO system is a means of typing human blood.
Blood contains many different antigens (commonly called factors): O, A, B, and Rh. These factors then combine to form blood types A, B, AB, and O with each of these types being either positive or negative for the Rh antigen. Accurate blood typing is essential when giving blood transfusions because some of these antigen combinations are not compatible with each other and can be fatal.
For instance, persons whose blood types are positive for Rh factor can be safely transfused with Rh negative blood of the same or compatible blood type while people who are Rh negative cannot be transfused with Rh positive blood. O negative blood is referred to as a "universal donor" because it can safely be transfused into anyone but can only accept transfusions from other O negative sources. On the other hand, people with AB positive blood can accept any type blood for transfusion, yet can only donate blood to other AB positives.
There is also a danger when an Rh negative mother is pregnant with an Rh positive baby.