Why is a person with blood group O called a universal donor and a person with blood group AB called a universal recipient?

A person with blood group O is called a universal donor since his blood can be given to a person with any blood group. As blood group O has neither antigen A nor B so it doesn't cause self-clumping with any antibodies making it compatible with all blood groups.
A person with blood group AB is called a universal recipient since he can receive blood from a person with any blood group. As blood group AB has no antibodies, so there is no attack on the antigens of any blood group making it compatible to receive blood from all blood groups.