Godmoding
also godmodding & god mode
Derived from the concept of your character obtaining god-like powers. Godmoding is when you control another character without their player's permission.
The simplest example is when deciding the outcome of events. Sandy can cast a spell at Joseph, but Sandy's player cannot write that the spell hit Joseph. That is for Joseph to decide.
Likewise, it is bad form to godmod thoughts and feelings of other characters, even when those characters are part of a crowd. Rebecca's player cannot write that every boy in the school lusts after her without making it clear that this is Rebecca's subjective opinion, because there will always be at least a few boys who don't care for Rebecca.
Powerplaying
Powerplaying occurs when a scene is manipulated so that one character is the most powerful. This could be because the character knows all the most powerful spells, but it could equally be because a character with less knowledge manages to avoid all harm.
Powerplaying is generally a consequence of players wanting to perform badass in character actions, but not wanting the consequences of those actions to fall back on their characters.
Metagaming
Metagaming is the most obvious of these three nightmares and the easiest to classify. It's simply when a player uses information that their character could or should not have had access to in character.
For example, say Bradley has been reading Hannah and Jillian's thread in Moaning Myrtle's Bathroom where they talk about a secret Hannah heard from Natalie.
In his next thread with Thomas, Bradley can't mention this little tidbit of information, since although his player knows about it, his character was not actually there to hear it.