Concrete sand is an aggregate sand usually composed of either gneiss, trap rock, limestone or granite.  Usually it is crushed at the Quarry and then washed through a screen to ensure that there are no large pieces of rocks left in it.
 
It is most commonly used as a key ingrediant in cement or hot asphalt but can also be used as pipe sand or as a base layer and leveling medium for above ground pools, and patios or walkways made of concrete paving stones.