A mob is a living game entity. Different mobs have different functions. There are many different types of mobs in rebirth of the night. Different mobs have different behaviors.
List of Mobs
Hostile mobs will attack the player on sight.
Neutral mobs will attack the player if provoked. Many are provoked by simply attacking them first but some have special ways of provoking them.
Passive mobs are harmless won't attack the player. Even when provoked or attacked first.
Boss mobs are special mobs which are exceptionally more tougher then regular mobs. Defeating them can lead to progression.