DM - Dungeon Master - A person who will normally guide a group of character's through a story or action. Used mostly as AD&D Terms.
IC - In character - The make believe character which is used in stories, plots, and Quests.
NPC - Non-Player Character - A general character which has no true name; example, A_walking_book. Or, a character on a scene which is usually only used to advance the RP; example, A bairmaid, a servant in a fort, a guard in a club.
OOC - Out of Character - The person who controls a Character or NPC.
PC - Player Character - Normally the term for a Character; however, some times it may be used to describe a Power Character (an extremely god like character).
ST - Storyteller - A person who will normally guide a group of character's through a story, chronicle, or scene. Used mostly in WoD terms.