no seriously, what is it?
There are certain antigens in your blood that define your bloodtype; it's a heredity trait.
O has no antigens. It can be donated to all.
A has "A" antigens, these will react with people of the B and O bloodtype.
B has "B" antigens, it will react with people of A and O.
AB has both antigens; and can be donated to by all.
Also, "reacting" defines clotting that leads to almost certain death.
RH factor is the "negative" or "positive." I forgot the true purpose of it, but basically if RH positive blood comes in contact with RH negative blood, it reacts. Roughly 25% of people have RH+.
Even if you have the right blood type, it can still clot.
so it's kind of trying your luck by getting a donation.