There are several different ways and reasons to give blood:

Whole blood donations. The most common blood donation. Each unit of donated blood is broken down into components:
Packed red cells are used to treat all kinds of anemia.
Platelet Concentrate is used to help clot blood and stop hemorrhaging.
Plasma is used for the treatment of clotting factor deficiencies, hemophilia, and shock.

Plateletpheresis. Platelets are only collected. Blood donors can donate 5 times as many platelets than a whole blood donation.

Plasmapheresis. Only plasma is collected. Plasma is the liquid part of the blood and is often used during organ transplants. Donors with AB blood types have "universal donor plasma" and are asked to donate plasma instead of whole blood.

Automated Pheresis Donations are used to collect a combination of red blood cells, plasma and/or platelets. These donations are designed to obtain the optimum mixture of blood components that can be collected based on the donors blood type and weight, and on the needs of the patients.

Pediatric Donations. Blood of donors who have donated several times and have blood type of O Positive or O negative, will be drawn in special bags to be used for pediatric transfusions. Low birth weight babies may need many small transfusions to treat anemia. One pediatric unit can be used for several days at a time.

Direct Donations are specified by a donor to be given to a specific recipient if the recipient's hospital is known. The units are used if the recipient is in need and if the donor and recipient are compatible.

Autologous Donations. Patients may donate for their own use in advance of elective surgery.
Information gathered from a booklet given to me today when I donated plasma to the Community Blood Centers of South Florida, Inc.