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.
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 obtained from defeating the creature.
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.