Jade can be something of a catch all term and means subtly different things in different parts of the world.
In the GIA system, true jade can only be Jadeite or Nephrite. Jadeite is far less common than nephrite and is considerably more expensive. Both Nephrite and jadeite are ultimately derived from Serpentinite which has been heavily altered by metasomatic processes. Most of the world's jade are found in locations which have a great deal of serpentinite and various forms of serpentinite are frequently described as jade.