Godmodding, originally called "godmoding", in reference to the "god mode" you can enter in a computer game to become invincible, is the bane of most RP venues and experienced roleplayers.
On many, if not most sites, doing it will get you muted, kicked, or banned.
Godmodding refers to any act of attempting to exert an unreasonable amount of influence over other characters, situations, or surroundings.
Specific forms of godmodding include the following:
Bunnying
Bunnying is taking control of characters other than your own in order to make the storyline go the way you want it to.
• Forming closed actions
A closed action can be defined as any action that cannot be avoided.
For instance, punching someone in the jaw (or anywhere... the point is specifying that the punch connects), stealing something from someone, turning someone into something else, or doing anything that in any way specifically affects another character without their consent.
This also includes things like pouncing on someone to hug them or otherwise show affection, folks! Just cause it is "nice" doesn't make it ok unless you know it is cool with the other player.
Rather than post that you are "doing" something, post the attempt to do whatever it is. This gives the other character(s) a chance to accept the action or avoid it.
Conversely, never allowing your character to be hit or affected by things that would realistically affect them is just about as bad, so don't do it! (See invulnerability and powergaming)
• Role Playing another character's actions / reactions without their consent.
Casting a spell on an area and watching everyone start to dance, releasing a poisonous gas, causing people to die/lose conciousness, or describing the expression on someone's face in response to something are all examples of this. In sexual roleplay, a common offence is describing someone else's level or signs of arousal; For instance, in response to a character saying she doesn't like something, saying that her (insert sign of arousal here) tells otherwise.
Unless you have gotten consent for something specific, or you are positive you know the player well enough to know that it will be ok, do not presume to describe another character's feelings, actions, expressions, thoughts, or anything else!
If your character knows their character well, your character might speculate on how someone is likely to react, but that is entirely different from dictating it.
Invulnerability
Playing a character who is completely immune to everything, or even nearly everything is a form of godmodding and is highly frowned upon, as well as forbidden in most RP venues.
No one, even most gods, is completely infallible.
Creating a character who cannot be harmed in any way, feels no pain, heals instantaneously, has endless stamina, etc, etc, is not only bad form, but is just plain stupid as well!
Even if your character is immortal, s/he should be able to feel pain and take damage. If your character can't feel pain, they they will be at risk for things like accidentally burning their feet off in the fire, or carelessly getting an arm lopped or ground off, because they won't feel it happening. If your character heals wounds instantly, it should use some of their body's resources, and weaken or tire them, and so on.
There is a very good, in depth guide to avoiding this particular no no here. It goes into great detail, and has lots of suggestions for creating powerful, yet believable characters.
Power Gaming
Power gaming, or powergaming, is the act of doing something impossible (i.e. godly) or unlikely for your character in order to gain advantage.
Some examples of power gaming include:
• Role Playing an action or ability without explaining its origin and/or nature.
If your character's bio or backstory does not include an ability and a reason for said ability, and the ability is not a well known common ability for your character's race, then your character should not have it. This especially includes pulling new abilities out of your nether regions for the sole purpose of manipulating RP!
• Performing actions outside of a preset range with no apparent or explained connections
e.g. a fire elemental causing water to lash at an opponent.
• Role Playing actions or abilities that, under the circumstances would be impossible
e.g. blocking a powerful attack when severely injured, blowing up the planet, or taking down an entire armed army bare-handed.
• Role Playing any actions that are outside your character's normal pattern of behaviour for the purpose of manipulating the RP
Metagaming
Metagaming refers to using any information or knowledge that your character does not have access to, to manipulate a RP situation.
Some examples of metagaming are:
• Omniscience
• Mind reading (without it being one of your character's existing abilities, AND taking shielding/blocking into account)
• Knowing anything about another character that your character has no reasonable way of knowing.
• Knowing information about a character before it’s been revealed unless your character and that character have a history which would explain your character knowing this.
• Using anything you have learned OOC (for instance, reading the character bio, OOC chatting, the character's thoughts or narrative regarding the character, etc) to gain an IC advantage in RP.
Basically, if your character has not witnessed, been told in character, or discovered something by realistic and reasonable means, then they do not know it, and you should not RP them doing anything which would require said knowledge!