The acronym NPC expands to Non-Player-Characters; traditionally beings controlled by the game system and not players. In Graal, this term has a much broader meaning and applies not only to human characters or monsters, but also to example decoration and weapons; basically anything that is scripted in GScript.
There are mainly three kinds of NPCs, NPC weapons which are held in the player's inventory, Local NPCs that belong to a level and Database NPCs that appear like Local NPCs but are treated specially.
NPC scripting is a key element of creating your own Playerworld.