NPC means non-personal character, or non-player character, that is, a character that is controlled by code, not by someone else playing the game. They usually look a little different compared to the player characters. You need to click on them to talk to them. Some NPCs buy and sell items, others give you quests, some take you to new places and back again, others give you clues, some heal you, others change something about your character, and some just say something to make you laugh. Below is a list (incomplete) of NPCs.

Name	Function
Ian	basic information
Neko	buy and sell
Ishi	monster points quest
Elanore	healing until level 11
Aidan	monster points quest
Vincent	bug legs quest
Sandra	red scorpion stingers quest
Doug	snake lamps quest
Ishyah	buy and sell
Taro	travel to snowland
Doctor	a clever joke
Xanith	sex change
Nicolas	hair dresser
Nicholas	iron ore quest