Blood typing is done before a person gets a blood transfusion. Blood typing is also commonly done before surgeries and pregnant women always have their blood typed as well. Blood is typed by antigens that are present on the surface of the blood cells. Blood group antigens are ABO and there are also Rh antigens.
The ABO test tells what blood type you are. These types are A, B, AB, or O. If you have type A blood, it means that you have the type A antigen. If you have type B blood, it means you have the type B antigen. If you have type AB, it means that you have both A and B antigens. If you have type O blood, it means you do not have the A or B antigen.
The Rh antigen may or may not be present on the blood cells. If the antigen is present you are considered to be Rh positive. If it is not present you are considered to be Rh negative.
Here is an example: Say your blood has been typed and the A antigen is present. In addition, you have the Rh antigen. This means that your blood type is A positive.
In general terms an antigen is any substance in the blood or body that the body recognizes as foreign. Antigens cause the immune system to produce antibodies to fight the foreign invaders. Antigens can be things like bacteria, viruses, pollen, or proteins.
In blood typing, or cross and type as they say in the field, meaning crossmatch and blood type, antigens are proteins that some blood types have but others do not have. For example, type A blood has A antigens, type O negative has no antigens. This is why type O negative blood is considered the "universal donor", because anyone can receive it by transfusion regardless of their specific blood type.