"MUD" stands for "Multi-User Dungeon (or Domain)". The concept is pretty old; MUDs have been around for 30+ years. Older gamers might think about a MUD as a "Multiplayer Zork."
When you connect to the server, you'll find no graphics - only text, leaving you to interact with the world through console and imagination. Many mudders insist that what the game-world looks like in their head is far better than technology can allow anyway. If you've never tried a text-based game, you might be surprised to discover it's a lot of fun. Want to go north? Type n. Want to look around? Type look. A large number of (usually) intuitive commands are available.
MUDs are generally very social environments. Often you will need the help of other players to team up against certain monsters and explore areas. Though not the path for everyone, Arctic provides game mechanics and incentives to be in a clan.