In Roleplaying, a Non Player Character (abbreviated in NPC) is a character pesent in the environemnts but not acted by one of the player. When required, the NPC are played by the Game masters.
On Second Life, NPC may be represented by prim objects, for example, guards in front of a door or the interactive NPC from the SWQS. They may also not be viewed in world, leaving the player to imagine their presence, for instance there is always jedi in the great temple, even if none is logged in world.
Even if Non Player Characters have a limited interaction, players has to consider them the same way they do with Player Characters.