
NPC stands for "Non Playerable Character". These types of characters are used in game for several different things. The most common are transportation (boat, carpet etc) characters, Shop Assistants, Bank Managers and Quest Riddlers.
NPC's respond to commands sent by players in game. An example would be to begin speaking with an NPC a tibia player would say "Hi". If a player then wanted to know what a shop assistant NPC sells he or she could say "Offers". This would trigger the NPC to respond with a list of commands for items the player can buy.
There are many NPC's in the Tibia Lands this category will show you which each does and what their job/ role is in tibia.