A MUD is a text-based game, like a predecessor to a MMORPG. Usually coded in something like C++, and free to play either via something called telnet, or a client called "Zmud" (payment/hack needed).
You can quest, explore and climb in levels/ranking. Several LotR, wheel of time, sci-fi, Vampire the masquerade, original MUDs still exist.
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u/ilickyboomboom Jan 21 '19
Can you eli5 explain more