Donating blood is much like having blood drawn-for the patient. The staff has much more to do. A tourniquet is placed around your arm and a vein located, blood begins to 'come out' and fill up a bag. When enough blood has filled the bag, the IV is removed from your arm, you eat or drink something -OR BOTH.
Like the other poster said, you might feel a bit dizzy for awhile, but this will pass. It is not abnormal at all and is the reason that they don't want you to leave until you feel fine! Easy.