Multiplaying - The act of making more than one account in a multiplayer/online game. This can be used to gain unfair advantages over legitimate players.
This is used by losers who have no lives outside of the game, and are willing to do whatever they can to gain advantage in the game.
Common uses of multiple accounts in games are:
Gaining the easy points in the beginning of game and giving money/items to the main account.
Using stronger/better account to be dummy account for weaker account (form adventure parties, where the stronger attacks a huge beast and sharing the experience with weaker).
Protecting a weaker account so that the weaker account never dies.
In some games having two similar worth accounts can give experience bonuses.
There are not any good ways to detect if someone is multiplaying. An obvious mistake for must multiplayers are having same or similar IP addresses. The player might also threaten another player not to attack his "alternate" account. This shows that this player is multiplaying. If you encounter such an instance, you should save the message, or log it and show it to an admin. Also, if some player that you never came into contact with suddenly attacks you without a reason, it is probably the alt of some other player.