When one plays a character that can never be destroyed, harmed, etc. The character, however, can hurt and/or kill the other characters without giving the other characters a chance.
This is highly illegal in most, if not all, roleplays.
Sarah = Regular player
Bob = Godmodder
-Sarah aims her arrows at Bob and then releases-
-Arrows go through Bob and Bob jumps up and reaches into Sarah's chest and drags out her beating heart, squeezing the life out of it and killing her-
Godmoding covers a lot more matter than usually given credit for. It's a term used in many forms of roleplaying (Be they table-top, forum, IM, etc.) for any of the following problems:
1) When a character knows information that they shouldn't - this information could be taken from a forum they aren't supposed to have accessed, the user's prior knowledge on a subject (usually in fanbased RPs that follow a timeline), outside conversations about characters, or anything along those lines.
2) When a player takes control of another player's character (this is negated if there is some form of mind control involved, but even still permission for actually moving the character should be discussed). A common, though often accidental, version of this is when a character attacks another character and they explain that the other character was hit, not giving that person a chance to let their character dodge.
3) When a character is somehow invincible - if there's a GM, good luck getting this character through to the end. Everyone has weaknesses, your character should be included.
Should anyone out there still not get it - Godmoding is a bad thing.
1) Situation - In another forum, the GM has had a hitman hired to assassinate Player A's character.
Hitman: *Happens to be a super pretty girl - just the kind that Player A's character likes* Hey baby.
Player A: *Shoots in the head* Stupid assassin.
2) Person: Player A shoots at Player B, hitting him twice in the arm and once in the knee. And then, Player B falls to the ground, crippled from pain.
- In which the person's character is Player A.
3) ... does this really need explaining? They're indestructible.
All of the above? Godmoding. Generally considered rude / illegal in play.
In a written RPG, this refers mostly to a player controlling another player's character without permission or trying to rewrite the plot to suit themselves. In written RPGs, the owner, ie the person who started the thread in the forum, has control of the plot unless he/she gives permission.
This is something often done by noobs.
(In an RPG set in the 50s)
Jane: Daisy walked into the diner and ordered a milkshake.
Noob: Immediately a man drove his tank into the front of the diner at warp speed, and started blasting off laser guns at everyone!
Jane: You idiot, that's godmoding!
(In the same RP a little later)
Jane: Daisy walked over to talk to her best friend, who was standing by the jukebox.
Noob: Daisy immediately slapped her friend.
Jane: Stop godmoding my character!
My character Tyan has unlimited lifes and can recover instantly from any attack!