A MUSH is a type of online roleplaying game. Strictly, MUSH is just a family of server software, based off of the MUDs that have been popular on the Internet for several decades now.
However, a MUSH is different from a MUD in a few important ways. Some of the commands and most of the coding is entirely different. As well, while a MUD will tend to include mobs, combat, guilds, and other hack-and-slash elements, a MUSH is almost exclusively dedicated to roleplaying.
So rather than go around smacking gophers with +1 vorpal swords, on From the Cradle we get our kicks from roleplaying in the older sense of the word; of acting and writing rather than beating. Not to say that there's no violence on From the Cradle because violence is a part of any society, but it is not the focus.