Lich is a scripting engine for text-based MUDs. It is not a complete front-end in and of itself: it operates much like a proxy server and communicates with the user through their chosen front-end (effectively giving the appearence of expanding the front-end's features with its own). Lich includes a slightly altered, extended, embedded-version of the Ruby interpreter.
This means it allows you to write and run scripts for a text-based MUD in the Ruby language. Why Ruby? Several reasons, really; first, moreso than any other language I've personally played with, Ruby accomodates you rather than trying to make you accomodate it. As a language I've found Ruby to possess an elegant self-sufficiency that abstracts away the need for a competent grasp of its low-level functioning, allowing one to simply make use of its features with very little need to be concerned with programming's more mundane details.
I could probably spend hours talking about the things I've come to love in Ruby, but in the interest of saving time, I can sum it up by saying two things: I used Ruby because it's a personal preference of mine, and because the interpreter is able to fit together so beautifully with an independently developed piece of software that it almost seems a shame not to take advantage of its extensible design.