Midnight Sun is a MUD. MUDs are text-based online multiplayer games with a rich and long history. They are the precursor of today's graphical MMO giants. And they refuse to die.
MUDs are text-based virtual worlds, and like books, depend on the players' imagination to visualize the game world. Players read text descriptions of locations (called 'rooms'), items, monsters, other players and NPCs and interact with them in real-time by typing text commands.
MUD games share a lot of characteristics with graphical MMORPGs – one way to think of them is as MMORPG games without graphics, and MMORPGs were sometimes also referred to as graphical MUDs. Some developers, such as Raph Koster (lead designer for Ultima Online, the first widely successful MMORPG), argue that they are the same thing, only with a different interface.
MUDs are played over a telnet connection and any telnet client is enough to connect to the game, but there is also a good selection of specialized MUD clients that improve the playing experience. Web-based clients which allow playing from a web browser also exist.
Most MUDs are run as hobbyist projects and are fully free to play, but there are also several commercial ones, either subscription based or pay-for-perks.
The term MUD stands for Multi-User Dungeon. It was also the name of the oldest game of this kind, developed in the late 1970's at the Essex University, which added a multiplayer aspect to the text-based adventure games of the time, such as Zork or Dungeon.
Over the time, as the genre diversified, the acronym also started to take on new meanings, such as multi-user dimension or domain, to accomodate projects that were more social or educational in nature. Different codebases were developed to suit a wide range of styles, from hack-and-slash through roleplay games, to games focused on socializing or world building.
The LPMud codebase that we built Midnight Sun on was developed in 1989 by Lars Pensjö. It uses an object-oriented scripting language called LPC, which provides a lot of flexibility and freedom in creating and populating game areas. All rooms, monsters, items and NPCs have to be hand-crafted, which creates a rich, detailed world without a 'generic' feel. This also extends to commands, which sometimes require more creativity on the part of the player and can make people used to more standardized codebases uncomfortable. Midnight Sun itself was started in 1991 and has been online and open to players ever since.
If you would like to know more about the history of MUDs, this Gamespy article from 2001 is well worth reading. And the Machinima video below goes through the earliest history of MUDs, as well as other early precursors of modern graphical online games: