Coco or Coir as it is somtimes know is one of the most popular growing medias. It is made up from coconut husks and is light and fluffy, holds very little nutrient which gives you a blank canvas to work with and is very well aerated.
Coco is more and more often being mixed with other growing media such as Clay Pebbles, Perlite and Growstones. These are used to help aerate the coco even more and can aid the uptake of water in certain systems.