Humans have mainly four blood groups. A, B,AB and O. Human blood contains antibodies that will trigger a reaction, if, during a blood donation, the donor is from a blood group that is different from that of the person receiving it. Therefore, before a blood transfusion, we have to 'type' a person's blood, and find out which kind it is.
Dogs have eight basic blood types, though as many as 13 may exist. The types are numbered, and are structured differently than in humans. Dogs do not have naturally occurring antibodies against other blood types.
Cats have three basic blood types. Cats do have natural antibodies against different blood types, like in humans. A cat can receive a blood transfusion only from a cat with the same blood type.