Thief

A Thief is someone that steals your Items, Money, Monsters, Loot Bag, etc.

Item/Money Thief
This is someone who uses a Scam or other method to get your good Equipment, Money, or other Items in a dishonest way.

For more information on avoiding Item/Money thieves, read the page on Scamming.

Loot Bag Thief
This is someone that grabs your Loot Bag while you are dragging it back to town. They may simply grab it while you are dragging it along, or they may throw some junk (Worm, Rubbish, etc.) or a single Gold Coin on it, so that instead of throwing your loot bag, you throw the item, then they have time to run over and grab the loot bag.

For tips on avoiding loot bag thieves, read the page on Loot Bags.

Monster/Creature Thief
This is someone that may follow you around, and then they will attack monsters/creatures that you have already begun attacking. They do this to steal some of the experience from defeating the creature, or to steal the loot from it when it is dead.

A variation on this is when you are training on one or two creatures, and another player attacks and kills the creatures you were training with. By doing this, they essentially get a "free" blocker.

Thief NPCs
To my knowledge, there is only one NPC that is a thief, and that is Aruda in Thais. When you are talking to her, if she says "Sorry, I was distracted", then she has just stolen some of your Gold.