Coco is made using the outer husk of the coconut shell. It's naturally rich in potassium and sodium, so most coco products are pre-treated by the manufactures so that they are ready to use. Coco is a relatively light medium that has good water retention without becoming soggy: it is much harder to over water coco than soil. It has an excellent air capacity of around 30% which aids root growth and nutrient uptake.
Coco contains a naturally occurring fungus called Trichoderma that helps prevent disease and aids root development. Due to gradual leaching of potassium from the coco it is strongly advised to use a coco-specific nutrient.