This article is a combination of “defining game terms” and “offer advice.” In games such as chess, minis, and cards, metagaming is being aware of the various strategies and styles and using the knowledge to gain an edge or defend against common attacks. It is perfectly acceptable, even expected. Think of movie scenes where the hero says “And I knew that you knew I knew, so I did this instead!”
In role-playing, metagaming is using out-of-character knowledge in game. Example: a GM is running an adventure module. One of his players has read it so he knows all the challenges and the plot. In the first session, his paladin kills the treacherous royal adviser despite having no in-character knowledge of the scheme. Such behavior is frowned upon as it often leads to power-gaming and puts the other players at a disadvantage. There are plenty of subtler methods as well.
There are exceptions, however. Times when metagaming is for the good of the game.
Polite metagaming. For example, if your character has mind control powers it might be tempting to use them against your fellow PCs. You might even feel justified as it’s what your character would do, say due to playing super-villains. It is an acceptable to metagame and not do so due to the fact it interferes with other players and may lead to the non-acceptable out-of-game grudge.
Punishing stupidity. Jokes and player disagreements are out-of-character, so it’s rude to punish a player through their character. However, saying things like, “The GM is too stupid to …” is asking for it. Really, it’s the player’s fault.
Plot wagon and willful genre blindness. This is accepting certain gaming conventions. For example, if two characters are killed, usually the two players will make new ones. The GM will then find a way to introduce the new characters into the adventure. This makes perfect sense from a game perceptive. From a story perceptive, it makes little sense for the surviving party members to accept random strangers into the fold, especially mere days after the deaths of their two comrades. Heck, just having the party work together can stretch suspension of disbelief. Nevertheless, playing with your pals demands it.
I’m sure there are other examples. Anyone have one they want to share?