This is designed for people who have no idea what this muck thing is; if you've visted mucks, mushes, moos, or muds before, you can probably skip it.
Good question. A muck is a text based interactive environment, sort of a low budget virtual reality. The text adventure games of the '80s, such as the ones that Infocom made, were similar--but with the differences that multiple players can move around, talking and role playing, and that it's even possible to expand the game universe on your own.
Yes, although the differences aren't very important to a new user. A MUD is a Multi-User Dungeon, and they tend to be more like games than chat or social interaction oriented systems. A MUCK (Multi-User Chat Kingdom) is almost always for people to have social interactions. There are also MUSHes, MOOs, and so on, but again the differences between them and MUCKs aren't likely to be obvious to someone coming in from IRCs.
You need a computer, a modem, and an ISP. Anyone who can use telnet can visit a muck. If you're reading this, you can visit FictionMuck. (Actually, I've discoverd that WebTV may not support telnet links so it's at least theoretically possible to be able to read this but not be able to visit a muck.)
Many muckers use TinyFugue, which is a program that allows multiple telnet connections to be used at the same time. It also supports other features such as macros; it is a very powerful program when used fully. (A tutorial can teach you how.) It is freeware, written by Ken Keys (hawkeye@tcp.com) and you can download tf yourself if your site doesn't already have it.
Well...not a lot, honestly. They're pretty intuitive and self-explanitory for the basic things, and help is available both from tutorial screens and from other users. (Many mucks, including FictionMuck, have helpstaff players who are experienced at solving problems.)
The basic commands are pretty simple; here are the ones that are used most and that a visitor will need. You can read a better guide to mucks for more elaborate explanations.
Get a beginner's tutorial on using mucks (from FictionMuck)