Multi-User Dungeon, a early form of text-based RPG, influenced by Roleplaying board games such as Dungeons and Dragons.
Prominent features:
1.Stat-System and Levels.
2.Quests and Events.
3.Inventories and Equipment system.
4.Multi-user chat
5.Class systems and early concepts of professions/specialization,
6.Chance-based combat

User-content: Although content in most MUDs was designed by staff or developers, a few variants(MUCK/MUSH/MOO) allowed users to create their own content(such MUDs are not-RPG based but roleplay-based chat, like an early precursor to SecondLife )

Influence: MUDs were instrumental in early video game evolution: RPG genre is especially designed from system refined in MUD/MUSH clients(e.g. King Quest series which is essentially a single-player MUD).
More info: