What blood types are there and how are they determined?

There are 4 basic blood types. A, B, AB, and O. Then, there is the "rh" factor, determining if you are negative or positive. So, there is A+, A-, B+, B-, AB+, AB-, and O+, or O-. "O" is typically the universal donor. That means that it is able to be taken by the other blood types IF you match the (+) or (-). AB- is the hardest type to find, and is less than 1% of the population.