This is highly illegal in most, if not all, roleplays.
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-
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.
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.
This is something often done by noobs.
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!