PlateletPlatelets are cellss that stick together to form blood clots. Normally they don't stick together, but when there is a disturbance in the blood vessels (such as a broken vessel) they stick together at that point and prevent blood from coming out of the vein or artery. They are not cells in the conventional sense, but are broken-off pieces of megakaryocyte cytoplasm released from the bone marrow into the blood stream.
Platelets are separated, by centrifugation, from the rest of donated blood and given to patients who need them. A (see-through) bag of them is pale orange. They are separated because they don't survive the normal storage used for red blood cells, and must be stored separately.
A normal platelet count in a healthy person is between 150 and 400 (x 109/L of blood). People can live independently with a count as low as 20. People can live in hospital with a count as low as 5, but spontaneous bleeding gets to be a problem. That's when platelet transfusions have to be done. A low platelet count is called thrombocytopenia, having too many platelets is called thrombocytosis.
See Also: Haemostasis