MOOs & MUDs

MUD (Multi-User Dungeon)
MUDs combine RPG's, hack and slashers, & PVP. Players can view descriptions of, and interact with, rooms, objects, other players,&  non-player characters. Players interact with the world by typing commands..

Traditional MUDs are role-playing video games set in a world populated by local races and monsters, be it Zombies, Humans, Alien or Elves, with players choosing classes in order to gain specific skills or powers. The objective of the game is usually to slay enemies, explore, complete quests, go on adventures, create a story by roleplaying, and advance the created character. Many MUD's have a main quest-line to follow, although some are more free flowing, with no real designated goal.

MOO
A MOO is a text-based online program in which multiple users interact at one time. Unlike MUD's, MOO's are more about filling a self-designated role within the world than leveling up and completing quests. Players can become traders, mechanics, blacksmiths, or mercenaries if they so choose.