There are two types of antibodies...Anti-A and Anti-B.
If you are type A blood, you have anti-B antibodies. Exposure to B blood is instant blood clotting (coagulation) and death.
If you are type B blood, you have anti-A antibodies. Exposure to A blood is instant blood clotting (coagulation) and death.
This is coagulation.
If you are type O blood, you have anti-A and anti-B. Exposure to A, B, or AB is coagulation. However, A, B, or AB people can receive your blood because you have their antibodies.
If you are AB blood, you do not have the anti-A and anti-B antibodies. You are a universal recipient.
The + or - is a Rhesus factor. Anyone with "-" that is exposed to "+" will start making the Rh antibodies, which is dangerous in the case of a Rh- mother and a Rh+ fetus. The mom will attack her baby because the body thinks that it is an intruder because of the presence of the antigen.
All blood types are inherited. You can actually determine your likelihood of having a blood type or your children having a blood type using a Punnett Square.
My mom is A, my dad is B. I am O. This means that my mom had to have an A allelle and an O, and my dad had a B and an O. AO x BO = 25% chance of having a baby with OO.
The blood type O is a recessive allele. A and B are dominant. This doesn't mean they're "more common," just that when they are present in the big old gene pool, they'll show.
I have known a handful of people with AB+ and they were all Eastern European Jews. I think that it is possible that they emigrated all from the same area. It's a fairly rare bloodtype, only about 4% of the population. There are Native American populations that are 100% O+. A+ is more common in European peoples, and B+ is overwhelmingly dominant in Asian populations.