Passive mobs are harmless mobs that do not attack the player, even when provoked or attacked. Some of them are even breedable.
Neutral mobs are sometimes passive and sometimes hostile towards the player. Many neutral mobs only become hostile when attacked first, though others have other ways they can be provoked. Some may be naturally hostile with provocation.
Hostile mobs are dangerous, aggressive mobs that always attack the player on sight.
Unused mobs do not spawn naturally in-game but are functionally in game. They can be spawned only with the /summon command, or sometimes with Spawn Eggs.
Removed mobs no longer exist in current versions of certain mods.