On a
MUD, a mobile is an NPC, a Non-Player Character.
Mobiles, usually called mobs can be monsters to kill, sages who give you quests, shopkeepers, trainers, any kind of creature you might interact with. Mobs are essentially little programs that compile into a text phrase which describes the creature and its equipment. Mobs may have code strings called mob_progs which will allow the mob to interact with the player in a number of ways, depending on variables such as the player level, race, alignment, tribe, size and health, among others. Some mob_progs are text activated, and require the player to speak a certain phrase or give a certain item to the mob in order to activate the progam. Some mobs become so intrinsic to the character of the MUD that they are treated almost as players, being a part of the folklore and feel of the mud. Sometimes they are even invited to parties!
These comments relate most specifically to Mirkwood MUD, mirkwood.serverstation.com port 4000. I'm Sard Rocklifter over there.