A MUCK is a text-based computer-run communal world, created and maintained by those who play on it. Players "log in" to characters they design which then move about in the game world. Characters in the game can speak to each other, move, handle, and create objects, build houses, meadows, or ski resorts, and can change themselves to become whatever sort of being they'd like. MUCKs often have themes, such as "furry", anime, or Gothic, and the characters and the buildings in the MUCK are arranged to suit the theme.
Some help files are available for new MUCKers:
More information on MUCKs and MUDs can be found at the MUD Resource Collection.