Stealing
The stealing skill can be a very effective way to make money, although you will not become very popular doing it. It is used to steal items from another player's backpack, although you cannot steal items that are within bags or that are newbiefied. Stealing will massively lower your karma and successfully stealing will make you criminal.
How to steal
First, you need to look inside another player's backpack using the Snooping skill, by double clicking on their backpack on their paperdoll which can be seen by double clicking the player. Once you can see inside their backpack, use the steal skill by clicking the button next to stealing on the skill menu, and click on an item to attempt steal. Heavier items are harder to steal and need higher stealing in order to steal them.