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