While humans share many similarities, we don't all have the same blood. Knowing your blood type is vital if you need a blood transfusion. If your body is not compatible with the blood you've been given, your body will reject it.
The differences in our blood start in the immune system. All of the cells in our bodies are covered in antigens. Antigens are a toxin or foreign substance that induce the production of antibodies. When your immune system detects antigens that aren't familiar, it triggers the release of antibodies that tag foreign substances to be destroyed.
There are two main types of antigens that form on the surface of every red blood cell: agglutinogens A and agglutinogens B. The type of agglutinogens present on the cells determine the blood type of a person. The cells can have both, only one, or neither. Having these antigens on your blood cells means you don't have antibodies for them. If you don't have the antigens, that means you have antibodies for them. So, for example, if you have red blood cells with A antigen on them, your blood type is A and your antibodies would attack B.
There's another set of antigens on your blood cells called the rhesus, or RH, system. These antigens work in different ways than the agglutinogens. The RH system is actually a collection of different antigens. They're produced as a single group, so you either have all 45 of them or none of them. If you have these antigens, you're RH positive. If you don't have these antigens, you're RH negative.
When it comes to blood transfusions, blood compatibility is vital to prevent a life-threatening reaction. The compatibility has less to do with the letter you have and more to do with which letter you have antibodies for. So, people with type AB blood don't have antibodies for both A and B agglutinogens. So they can accept A, B, AB, and O blood. People with type O blood have both A and B antibodies, so they can only accept O.
As for the RH antigens, people with RH positive blood can accept either positive or negative blood. People with RH negative blood are safest taking only negative blood. They can, however, tolerate positive blood, but can only do so one time. After that, their bodies will form antibodies against the RH antigens they received.