Auto
To control a character that does not belong to you. Sometimes referred to as "god-moding."

Auto-Hit
To injure someone else's character without giving that person the chance to decide if the character dodges or something similar.

Char
Care
Short for "character."

Cookie Cutter Character
A character who is almost identical to another character belonging to the same player. (IE, cut from the same cookie cutter.)

Despie
Dess-pee
Short for "desperate," a despie is a character who is clearly desperate for some good lovin'. He or she is usually highly attractive, talented, and wears sexy clothing. Tell-tale signs include lines like "Single and looking!" in a description.

God-Mode
The definition seems to vary. In some circles, it means the same as autoing, while others liken it to power-playing.
Note - the term "godmoding" has been corrupted in some places to "godmodding." However, I believe "godmoding" - one D - is the correct term, as it makes more sense in the present tense. (How does one "god-mod," anyway?)

Mary-Sue
Sometimes, but not always an idealized version of the writer. Mary-Sue's traits include being beautiful, multitalented, and having a terribly tragic past. They are very often attached to a canon character somehow, being related or a love-interest. Often referred to as a Gary Stu, Harry Stu, or Marty Stu when male.

Meta-Play
To play a character so that he or she has knowledge that was not logically aquired.

Power-Play
To play a character so that it is too powerful in some respect, such as having city-shattering powers or being impervious to injury. Sometimes referred to as "god-moding."

RPG
Short for "Role-Playing Game." (Naturally, "RP" is short for "Role-Play.)