MUSH stands for multi-user shared hallucinations -- or something. A MUSH is a server one can telnet to and play a text-based role-playing game (a bit like the Infocom ones, if you are, um, mature enough to remember them) on with other people. For some reason, MUSHes seem to attract people that care for language, experience, and generally encountering new situations, while MUDs (mult-user dungeons) generally seem to attract the hack & slay crowd. This might be due to the fact that MUSHes have next to no support for fighting, an often straining character creation process and generally less options for "winning the game". Certain stats may be raised, but this is not the objective of the game. MUDs have four basic modes of conduct:

• team up with other player characters to kill bigger enemies
• fight other player characters
• ignore other players ICly, only talk to them OOCly ask for help on certain riddles, solve riddles alone, gain points.
• IC interaction with other player characters. This seems to be a rare and neglected option on most MUDs. On MUSHes, it is the only option. On MUDs, you can still go out and kill things or solve coded puzzles, while on MUSH, you're stuck. On MUSH, co-existing is not enough, playing while others are playing does not suffice; if you don't interact, the game is null and void.

IC vs OOC

OOC means "out of character", denoting all that relates to you, the player (or actor), not your character (or role).
IC means "in character", denoting everything relating to the role (or character) you play.
If Harrison Ford plays Deckard in "Bladerunner", Deckard is the role or character, while Ford is the actor. If Emma Rhoids plays Buffy, the Vampire Slayer, Emma is the player/actor, while Buffy is the role. What Emma says as Buffy is in character; what Emma might say for herself about how she cannot finish the scene because she has to go to work to be able to pay the internet bill is out of character.
->Player Character (PC)

To play a character, like an actor does in a stage play. The difference is, we don't normally work with scripts. We make up our responses to situations as they occur. Situations are formed by our own (player character) actions, those of other player characters and those of non-player characters controlled by players or staff. A good role-player would also take things like setting, weather, mood, et cetera into account --- we're "like writing this book together".

Non-Player Character (NPC or puppet)

A minor part (or semi-attributed ("statted") character) in the drama of the MUSH that is intended to enrich a given situation in role-play. NPCs can be created by players or by staff; they can be @created to physically exist as an object (often @set PUPPET) in the MUSH database, or they can be fully imaginary (using @emit). Shopkeepers, barmaids and other sidekicks are normally NPCs.
->PC

A leading-role (or fully "statted" character) in the drama of the MUSH; one that is intended to participate in role-play on a long-term "full-tilt" basis.
->NPC